Move mixed/js (#1383)

* Move mixed/js/core.js to js/core.js

* Move mixed/js/yomichan.js to js/yomichan.js

* Move mixed/js/timer.js to js/debug/timer.js

* Move mixed/js/hotkey-handler.js to js/input/hotkey-handler.js

* Move mixed/js/hotkey-help-controller.js to js/input/hotkey-help-controller.js

* Move mixed/js/hotkey-util.js to js/input/hotkey-util.js

* Move mixed/js/audio-system.js to js/input/audio-system.js

* Move mixed/js/media-loader.js to js/input/media-loader.js

* Move mixed/js/text-to-speech-audio.js to js/input/text-to-speech-audio.js

* Move mixed/js/comm.js to js/comm/cross-frame-api.js

* Move mixed/js/api.js to js/comm/api.js

* Move mixed/js/frame-client.js to js/comm/frame-client.js

* Move mixed/js/frame-endpoint.js to js/comm/frame-endpoint.js

* Move mixed/js/display.js to js/display/display.js

* Move mixed/js/display-audio.js to js/display/display-audio.js

* Move mixed/js/display-generator.js to js/display/display-generator.js

* Move mixed/js/display-history.js to js/display/display-history.js

* Move mixed/js/display-notification.js to js/display/display-notification.js

* Move mixed/js/display-profile-selection.js to js/display/display-profile-selection.js

* Move mixed/js/japanese.js to js/language/japanese-util.js

* Move mixed/js/dictionary-data-util.js to js/language/dictionary-data-util.js

* Move mixed/js/document-focus-controller.js to js/dom/document-focus-controller.js

* Move mixed/js/document-util.js to js/dom/document-util.js

* Move mixed/js/dom-data-binder.js to js/dom/dom-data-binder.js

* Move mixed/js/html-template-collection.js to js/dom/html-template-collection.js

* Move mixed/js/panel-element.js to js/dom/panel-element.js

* Move mixed/js/popup-menu.js to js/dom/popup-menu.js

* Move mixed/js/selector-observer.js to js/dom/selector-observer.js

* Move mixed/js/scroll.js to js/dom/window-scroll.js

* Move mixed/js/text-scanner.js to js/language/text-scanner.js

* Move mixed/js/cache-map.js to js/general/cache-map.js

* Move mixed/js/object-property-accessor.js to js/general/object-property-accessor.js

* Move mixed/js/task-accumulator.js to js/general/task-accumulator.js

* Move mixed/js/environment.js to js/background/environment.js

* Move mixed/js/dynamic-loader.js to js/scripting/dynamic-loader.js

* Move mixed/js/dynamic-loader-sentinel.js to js/scripting/dynamic-loader-sentinel.js

8 files changed, 1666 insertions, 0 deletions

diff --git a/ext/js/dom/document-focus-controller.js b/ext/js/dom/document-focus-controller.js
new file mode 100644
index 00000000..649b5abe
--- /dev/null
+++ b/ext/js/dom/document-focus-controller.js
@@ -0,0 +1,110 @@
+/*
+ * Copyright (C) 2020-2021  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/**
+ * This class is used to control the document focus when a non-body element contains the main scrollbar.
+ * Web browsers will not automatically focus a custom element with the scrollbar on load, which results in
+ * keyboard shortcuts (e.g. arrow keys) not controlling page scroll. Instead, this class will manually
+ * focus a dummy element inside the main content, which gives keyboard scroll focus to that element.
+ */
+class DocumentFocusController {
+    constructor() {
+        this._contentScrollFocusElement = document.querySelector('#content-scroll-focus');
+    }
+
+    prepare() {
+        window.addEventListener('focus', this._onWindowFocus.bind(this), false);
+        this._updateFocusedElement(false);
+    }
+
+    blurElement(element) {
+        if (document.activeElement !== element) { return; }
+        element.blur();
+        this._updateFocusedElement(false);
+    }
+
+    // Private
+
+    _onWindowFocus() {
+        this._updateFocusedElement(false);
+    }
+
+    _updateFocusedElement(force) {
+        const target = this._contentScrollFocusElement;
+        if (target === null) { return; }
+
+        const {activeElement} = document;
+        if (
+            force ||
+            activeElement === null ||
+            activeElement === document.documentElement ||
+            activeElement === document.body
+        ) {
+            // Get selection
+            const selection = window.getSelection();
+            const selectionRanges1 = this._getSelectionRanges(selection);
+
+            // Note: This function will cause any selected text to be deselected on Firefox.
+            target.focus({preventScroll: true});
+
+            // Restore selection
+            const selectionRanges2 = this._getSelectionRanges(selection);
+            if (!this._areRangesSame(selectionRanges1, selectionRanges2)) {
+                this._setSelectionRanges(selection, selectionRanges1);
+            }
+        }
+    }
+
+    _getSelectionRanges(selection) {
+        const ranges = [];
+        for (let i = 0, ii = selection.rangeCount; i < ii; ++i) {
+            ranges.push(selection.getRangeAt(i));
+        }
+        return ranges;
+    }
+
+    _setSelectionRanges(selection, ranges) {
+        selection.removeAllRanges();
+        for (const range of ranges) {
+            selection.addRange(range);
+        }
+    }
+
+    _areRangesSame(ranges1, ranges2) {
+        const ii = ranges1.length;
+        if (ii !== ranges2.length) {
+            return false;
+        }
+
+        for (let i = 0; i < ii; ++i) {
+            const range1 = ranges1[i];
+            const range2 = ranges2[i];
+            try {
+                if (
+                    range1.compareBoundaryPoints(Range.START_TO_START, range2) !== 0 ||
+                    range1.compareBoundaryPoints(Range.END_TO_END, range2) !== 0
+                ) {
+                    return false;
+                }
+            } catch (e) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+}
diff --git a/ext/js/dom/document-util.js b/ext/js/dom/document-util.js
new file mode 100644
index 00000000..513a0c05
--- /dev/null
+++ b/ext/js/dom/document-util.js
@@ -0,0 +1,570 @@
+/*
+ * Copyright (C) 2016-2021  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/* global
+ * DOMTextScanner
+ * TextSourceElement
+ * TextSourceRange
+ */
+
+class DocumentUtil {
+    constructor() {
+        this._transparentColorPattern = /rgba\s*\([^)]*,\s*0(?:\.0+)?\s*\)/;
+    }
+
+    getRangeFromPoint(x, y, deepContentScan) {
+        const elements = this._getElementsFromPoint(x, y, deepContentScan);
+        let imposter = null;
+        let imposterContainer = null;
+        let imposterSourceElement = null;
+        if (elements.length > 0) {
+            const element = elements[0];
+            switch (element.nodeName.toUpperCase()) {
+                case 'IMG':
+                case 'BUTTON':
+                    return new TextSourceElement(element);
+                case 'INPUT':
+                    imposterSourceElement = element;
+                    [imposter, imposterContainer] = this._createImposter(element, false);
+                    break;
+                case 'TEXTAREA':
+                    imposterSourceElement = element;
+                    [imposter, imposterContainer] = this._createImposter(element, true);
+                    break;
+            }
+        }
+
+        const range = this._caretRangeFromPointExt(x, y, deepContentScan ? elements : []);
+        if (range !== null) {
+            if (imposter !== null) {
+                this._setImposterStyle(imposterContainer.style, 'z-index', '-2147483646');
+                this._setImposterStyle(imposter.style, 'pointer-events', 'none');
+            }
+            return new TextSourceRange(range, '', imposterContainer, imposterSourceElement);
+        } else {
+            if (imposterContainer !== null) {
+                imposterContainer.parentNode.removeChild(imposterContainer);
+            }
+            return null;
+        }
+    }
+
+    /**
+     * Extract a sentence from a document.
+     * @param source The text source object, either `TextSourceRange` or `TextSourceElement`.
+     * @param layoutAwareScan Whether or not layout-aware scan mode should be used.
+     * @param extent The length of the sentence to extract.
+     * @param terminatorMap A mapping of characters that terminate a sentence.
+     *   Format:
+     *   ```js
+     *   new Map([ [character: string, [includeCharacterAtStart: boolean, includeCharacterAtEnd: boolean]], ... ])
+     *   ```
+     * @param forwardQuoteMap A mapping of quote characters that delimit a sentence.
+     *   Format:
+     *   ```js
+     *   new Map([ [character: string, [otherCharacter: string, includeCharacterAtStart: boolean]], ... ])
+     *   ```
+     * @param backwardQuoteMap A mapping of quote characters that delimit a sentence,
+     *   which is the inverse of forwardQuoteMap.
+     *   Format:
+     *   ```js
+     *   new Map([ [character: string, [otherCharacter: string, includeCharacterAtEnd: boolean]], ... ])
+     *   ```
+     * @returns The sentence and the offset to the original source: `{sentence: string, offset: integer}`.
+     */
+    extractSentence(source, layoutAwareScan, extent, terminatorMap, forwardQuoteMap, backwardQuoteMap) {
+        // Scan text
+        source = source.clone();
+        const startLength = source.setStartOffset(extent, layoutAwareScan);
+        const endLength = source.setEndOffset(extent * 2 - startLength, layoutAwareScan, true);
+        const text = source.text();
+        const textLength = text.length;
+        const textEndAnchor = textLength - endLength;
+        let pos1 = startLength;
+        let pos2 = textEndAnchor;
+
+        // Move backward
+        let quoteStack = [];
+        for (; pos1 > 0; --pos1) {
+            const c = text[pos1 - 1];
+            if (c === '\n') { break; }
+
+            if (quoteStack.length === 0) {
+                const terminatorInfo = terminatorMap.get(c);
+                if (typeof terminatorInfo !== 'undefined') {
+                    if (terminatorInfo[0]) { --pos1; }
+                    break;
+                }
+            }
+
+            let quoteInfo = forwardQuoteMap.get(c);
+            if (typeof quoteInfo !== 'undefined') {
+                if (quoteStack.length === 0) {
+                    if (quoteInfo[1]) { --pos1; }
+                    break;
+                } else if (quoteStack[0] === c) {
+                    quoteStack.pop();
+                    continue;
+                }
+            }
+
+            quoteInfo = backwardQuoteMap.get(c);
+            if (typeof quoteInfo !== 'undefined') {
+                quoteStack.unshift(quoteInfo[0]);
+            }
+        }
+
+        // Move forward
+        quoteStack = [];
+        for (; pos2 < textLength; ++pos2) {
+            const c = text[pos2];
+            if (c === '\n') { break; }
+
+            if (quoteStack.length === 0) {
+                const terminatorInfo = terminatorMap.get(c);
+                if (typeof terminatorInfo !== 'undefined') {
+                    if (terminatorInfo[1]) { ++pos2; }
+                    break;
+                }
+            }
+
+            let quoteInfo = backwardQuoteMap.get(c);
+            if (typeof quoteInfo !== 'undefined') {
+                if (quoteStack.length === 0) {
+                    if (quoteInfo[1]) { ++pos2; }
+                    break;
+                } else if (quoteStack[0] === c) {
+                    quoteStack.pop();
+                    continue;
+                }
+            }
+
+            quoteInfo = forwardQuoteMap.get(c);
+            if (typeof quoteInfo !== 'undefined') {
+                quoteStack.unshift(quoteInfo[0]);
+            }
+        }
+
+        // Trim whitespace
+        for (; pos1 < startLength && this._isWhitespace(text[pos1]); ++pos1) { /* NOP */ }
+        for (; pos2 > textEndAnchor && this._isWhitespace(text[pos2 - 1]); --pos2) { /* NOP */ }
+
+        // Result
+        return {
+            text: text.substring(pos1, pos2),
+            offset: startLength - pos1
+        };
+    }
+
+    static isPointInRect(x, y, rect) {
+        return (
+            x >= rect.left && x < rect.right &&
+            y >= rect.top && y < rect.bottom
+        );
+    }
+
+    static isPointInAnyRect(x, y, rects) {
+        for (const rect of rects) {
+            if (this.isPointInRect(x, y, rect)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    static isPointInSelection(x, y, selection) {
+        for (let i = 0; i < selection.rangeCount; ++i) {
+            const range = selection.getRangeAt(i);
+            if (this.isPointInAnyRect(x, y, range.getClientRects())) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    static isMouseButtonPressed(mouseEvent, button) {
+        const mouseEventButton = mouseEvent.button;
+        switch (button) {
+            case 'primary': return mouseEventButton === 0;
+            case 'secondary': return mouseEventButton === 2;
+            case 'auxiliary': return mouseEventButton === 1;
+            default: return false;
+        }
+    }
+
+    static getActiveModifiers(event) {
+        const modifiers = [];
+        if (event.altKey) { modifiers.push('alt'); }
+        if (event.ctrlKey) { modifiers.push('ctrl'); }
+        if (event.metaKey) { modifiers.push('meta'); }
+        if (event.shiftKey) { modifiers.push('shift'); }
+        return modifiers;
+    }
+
+    static getActiveModifiersAndButtons(event) {
+        const modifiers = this.getActiveModifiers(event);
+        this._getActiveButtons(event, modifiers);
+        return modifiers;
+    }
+
+    static getActiveButtons(event) {
+        const buttons = [];
+        this._getActiveButtons(event, buttons);
+        return buttons;
+    }
+
+    static addFullscreenChangeEventListener(onFullscreenChanged, eventListenerCollection=null) {
+        const target = document;
+        const options = false;
+        const fullscreenEventNames = [
+            'fullscreenchange',
+            'MSFullscreenChange',
+            'mozfullscreenchange',
+            'webkitfullscreenchange'
+        ];
+        for (const eventName of fullscreenEventNames) {
+            if (eventListenerCollection === null) {
+                target.addEventListener(eventName, onFullscreenChanged, options);
+            } else {
+                eventListenerCollection.addEventListener(target, eventName, onFullscreenChanged, options);
+            }
+        }
+    }
+
+    static getFullscreenElement() {
+        return (
+            document.fullscreenElement ||
+            document.msFullscreenElement ||
+            document.mozFullScreenElement ||
+            document.webkitFullscreenElement ||
+            null
+        );
+    }
+
+    static getNodesInRange(range) {
+        const end = range.endContainer;
+        const nodes = [];
+        for (let node = range.startContainer; node !== null; node = this.getNextNode(node)) {
+            nodes.push(node);
+            if (node === end) { break; }
+        }
+        return nodes;
+    }
+
+    static getNextNode(node) {
+        let next = node.firstChild;
+        if (next === null) {
+            while (true) {
+                next = node.nextSibling;
+                if (next !== null) { break; }
+
+                next = node.parentNode;
+                if (next === null) { break; }
+
+                node = next;
+            }
+        }
+        return next;
+    }
+
+    static anyNodeMatchesSelector(nodes, selector) {
+        const ELEMENT_NODE = Node.ELEMENT_NODE;
+        for (let node of nodes) {
+            for (; node !== null; node = node.parentNode) {
+                if (node.nodeType !== ELEMENT_NODE) { continue; }
+                if (node.matches(selector)) { return true; }
+                break;
+            }
+        }
+        return false;
+    }
+
+    static everyNodeMatchesSelector(nodes, selector) {
+        const ELEMENT_NODE = Node.ELEMENT_NODE;
+        for (let node of nodes) {
+            while (true) {
+                if (node === null) { return false; }
+                if (node.nodeType === ELEMENT_NODE && node.matches(selector)) { break; }
+                node = node.parentNode;
+            }
+        }
+        return true;
+    }
+
+    static isMetaKeySupported(os, browser) {
+        return !(browser === 'firefox' || browser === 'firefox-mobile') || os === 'mac';
+    }
+
+    static _getActiveButtons(event, array) {
+        let {buttons} = event;
+        if (typeof buttons === 'number' && buttons > 0) {
+            for (let i = 0; i < 6; ++i) {
+                const buttonFlag = (1 << i);
+                if ((buttons & buttonFlag) !== 0) {
+                    array.push(`mouse${i}`);
+                    buttons &= ~buttonFlag;
+                    if (buttons === 0) { break; }
+                }
+            }
+        }
+    }
+
+    // Private
+
+    _setImposterStyle(style, propertyName, value) {
+        style.setProperty(propertyName, value, 'important');
+    }
+
+    _createImposter(element, isTextarea) {
+        const body = document.body;
+        if (body === null) { return [null, null]; }
+
+        const elementStyle = window.getComputedStyle(element);
+        const elementRect = element.getBoundingClientRect();
+        const documentRect = document.documentElement.getBoundingClientRect();
+        let left = elementRect.left - documentRect.left;
+        let top = elementRect.top - documentRect.top;
+
+        // Container
+        const container = document.createElement('div');
+        const containerStyle = container.style;
+        this._setImposterStyle(containerStyle, 'all', 'initial');
+        this._setImposterStyle(containerStyle, 'position', 'absolute');
+        this._setImposterStyle(containerStyle, 'left', '0');
+        this._setImposterStyle(containerStyle, 'top', '0');
+        this._setImposterStyle(containerStyle, 'width', `${documentRect.width}px`);
+        this._setImposterStyle(containerStyle, 'height', `${documentRect.height}px`);
+        this._setImposterStyle(containerStyle, 'overflow', 'hidden');
+        this._setImposterStyle(containerStyle, 'opacity', '0');
+        this._setImposterStyle(containerStyle, 'pointer-events', 'none');
+        this._setImposterStyle(containerStyle, 'z-index', '2147483646');
+
+        // Imposter
+        const imposter = document.createElement('div');
+        const imposterStyle = imposter.style;
+
+        let value = element.value;
+        if (value.endsWith('\n')) { value += '\n'; }
+        imposter.textContent = value;
+
+        for (let i = 0, ii = elementStyle.length; i < ii; ++i) {
+            const property = elementStyle[i];
+            this._setImposterStyle(imposterStyle, property, elementStyle.getPropertyValue(property));
+        }
+        this._setImposterStyle(imposterStyle, 'position', 'absolute');
+        this._setImposterStyle(imposterStyle, 'top', `${top}px`);
+        this._setImposterStyle(imposterStyle, 'left', `${left}px`);
+        this._setImposterStyle(imposterStyle, 'margin', '0');
+        this._setImposterStyle(imposterStyle, 'pointer-events', 'auto');
+
+        if (isTextarea) {
+            if (elementStyle.overflow === 'visible') {
+                this._setImposterStyle(imposterStyle, 'overflow', 'auto');
+            }
+        } else {
+            this._setImposterStyle(imposterStyle, 'overflow', 'hidden');
+            this._setImposterStyle(imposterStyle, 'white-space', 'nowrap');
+            this._setImposterStyle(imposterStyle, 'line-height', elementStyle.height);
+        }
+
+        container.appendChild(imposter);
+        body.appendChild(container);
+
+        // Adjust size
+        const imposterRect = imposter.getBoundingClientRect();
+        if (imposterRect.width !== elementRect.width || imposterRect.height !== elementRect.height) {
+            const width = parseFloat(elementStyle.width) + (elementRect.width - imposterRect.width);
+            const height = parseFloat(elementStyle.height) + (elementRect.height - imposterRect.height);
+            this._setImposterStyle(imposterStyle, 'width', `${width}px`);
+            this._setImposterStyle(imposterStyle, 'height', `${height}px`);
+        }
+        if (imposterRect.x !== elementRect.x || imposterRect.y !== elementRect.y) {
+            left += (elementRect.left - imposterRect.left);
+            top += (elementRect.top - imposterRect.top);
+            this._setImposterStyle(imposterStyle, 'left', `${left}px`);
+            this._setImposterStyle(imposterStyle, 'top', `${top}px`);
+        }
+
+        imposter.scrollTop = element.scrollTop;
+        imposter.scrollLeft = element.scrollLeft;
+
+        return [imposter, container];
+    }
+
+    _getElementsFromPoint(x, y, all) {
+        if (all) {
+            // document.elementsFromPoint can return duplicates which must be removed.
+            const elements = document.elementsFromPoint(x, y);
+            return elements.filter((e, i) => elements.indexOf(e) === i);
+        }
+
+        const e = document.elementFromPoint(x, y);
+        return e !== null ? [e] : [];
+    }
+
+    _isPointInRange(x, y, range) {
+        // Require a text node to start
+        if (range.startContainer.nodeType !== Node.TEXT_NODE) {
+            return false;
+        }
+
+        // Scan forward
+        const nodePre = range.endContainer;
+        const offsetPre = range.endOffset;
+        try {
+            const {node, offset, content} = new DOMTextScanner(range.endContainer, range.endOffset, true, false).seek(1);
+            range.setEnd(node, offset);
+
+            if (!this._isWhitespace(content) && DocumentUtil.isPointInAnyRect(x, y, range.getClientRects())) {
+                return true;
+            }
+        } finally {
+            range.setEnd(nodePre, offsetPre);
+        }
+
+        // Scan backward
+        const {node, offset, content} = new DOMTextScanner(range.startContainer, range.startOffset, true, false).seek(-1);
+        range.setStart(node, offset);
+
+        if (!this._isWhitespace(content) && DocumentUtil.isPointInAnyRect(x, y, range.getClientRects())) {
+            // This purposefully leaves the starting offset as modified and sets the range length to 0.
+            range.setEnd(node, offset);
+            return true;
+        }
+
+        // No match
+        return false;
+    }
+
+    _isWhitespace(string) {
+        return string.trim().length === 0;
+    }
+
+    _caretRangeFromPoint(x, y) {
+        if (typeof document.caretRangeFromPoint === 'function') {
+            // Chrome, Edge
+            return document.caretRangeFromPoint(x, y);
+        }
+
+        if (typeof document.caretPositionFromPoint === 'function') {
+            // Firefox
+            return this._caretPositionFromPoint(x, y);
+        }
+
+        // No support
+        return null;
+    }
+
+    _caretPositionFromPoint(x, y) {
+        const position = document.caretPositionFromPoint(x, y);
+        if (position === null) {
+            return null;
+        }
+        const node = position.offsetNode;
+        if (node === null) {
+            return null;
+        }
+
+        const range = document.createRange();
+        const offset = (node.nodeType === Node.TEXT_NODE ? position.offset : 0);
+        try {
+            range.setStart(node, offset);
+            range.setEnd(node, offset);
+        } catch (e) {
+            // Firefox throws new DOMException("The operation is insecure.")
+            // when trying to select a node from within a ShadowRoot.
+            return null;
+        }
+        return range;
+    }
+
+    _caretRangeFromPointExt(x, y, elements) {
+        const modifications = [];
+        try {
+            let i = 0;
+            let startContinerPre = null;
+            while (true) {
+                const range = this._caretRangeFromPoint(x, y);
+                if (range === null) {
+                    return null;
+                }
+
+                const startContainer = range.startContainer;
+                if (startContinerPre !== startContainer) {
+                    if (this._isPointInRange(x, y, range)) {
+                        return range;
+                    }
+                    startContinerPre = startContainer;
+                }
+
+                i = this._disableTransparentElement(elements, i, modifications);
+                if (i < 0) {
+                    return null;
+                }
+            }
+        } finally {
+            if (modifications.length > 0) {
+                this._restoreElementStyleModifications(modifications);
+            }
+        }
+    }
+
+    _disableTransparentElement(elements, i, modifications) {
+        while (true) {
+            if (i >= elements.length) {
+                return -1;
+            }
+
+            const element = elements[i++];
+            if (this._isElementTransparent(element)) {
+                const style = element.hasAttribute('style') ? element.getAttribute('style') : null;
+                modifications.push({element, style});
+                element.style.setProperty('pointer-events', 'none', 'important');
+                return i;
+            }
+        }
+    }
+
+    _restoreElementStyleModifications(modifications) {
+        for (const {element, style} of modifications) {
+            if (style === null) {
+                element.removeAttribute('style');
+            } else {
+                element.setAttribute('style', style);
+            }
+        }
+    }
+
+    _isElementTransparent(element) {
+        if (
+            element === document.body ||
+            element === document.documentElement
+        ) {
+            return false;
+        }
+        const style = window.getComputedStyle(element);
+        return (
+            parseFloat(style.opacity) <= 0 ||
+            style.visibility === 'hidden' ||
+            (style.backgroundImage === 'none' && this._isColorTransparent(style.backgroundColor))
+        );
+    }
+
+    _isColorTransparent(cssColor) {
+        return this._transparentColorPattern.test(cssColor);
+    }
+}
diff --git a/ext/js/dom/dom-data-binder.js b/ext/js/dom/dom-data-binder.js
new file mode 100644
index 00000000..292b2f67
--- /dev/null
+++ b/ext/js/dom/dom-data-binder.js
@@ -0,0 +1,234 @@
+/*
+ * Copyright (C) 2020-2021  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/* global
+ * SelectorObserver
+ * TaskAccumulator
+ */
+
+class DOMDataBinder {
+    constructor({selector, ignoreSelectors=[], createElementMetadata, compareElementMetadata, getValues, setValues, onError=null}) {
+        this._selector = selector;
+        this._ignoreSelectors = ignoreSelectors;
+        this._createElementMetadata = createElementMetadata;
+        this._compareElementMetadata = compareElementMetadata;
+        this._getValues = getValues;
+        this._setValues = setValues;
+        this._onError = onError;
+        this._updateTasks = new TaskAccumulator(this._onBulkUpdate.bind(this));
+        this._assignTasks = new TaskAccumulator(this._onBulkAssign.bind(this));
+        this._selectorObserver = new SelectorObserver({
+            selector,
+            ignoreSelector: (ignoreSelectors.length > 0 ? ignoreSelectors.join(',') : null),
+            onAdded: this._createObserver.bind(this),
+            onRemoved: this._removeObserver.bind(this),
+            onChildrenUpdated: this._onObserverChildrenUpdated.bind(this),
+            isStale: this._isObserverStale.bind(this)
+        });
+    }
+
+    observe(element) {
+        this._selectorObserver.observe(element, true);
+    }
+
+    disconnect() {
+        this._selectorObserver.disconnect();
+    }
+
+    async refresh() {
+        await this._updateTasks.enqueue(null, {all: true});
+    }
+
+    // Private
+
+    async _onBulkUpdate(tasks) {
+        let all = false;
+        const targets = [];
+        for (const [observer, task] of tasks) {
+            if (observer === null) {
+                if (task.data.all) {
+                    all = true;
+                    break;
+                }
+            } else {
+                targets.push([observer, task]);
+            }
+        }
+        if (all) {
+            targets.length = 0;
+            for (const observer of this._selectorObserver.datas()) {
+                targets.push([observer, null]);
+            }
+        }
+
+        const args = targets.map(([observer]) => ({
+            element: observer.element,
+            metadata: observer.metadata
+        }));
+        const responses = await this._getValues(args);
+        this._applyValues(targets, responses, true);
+    }
+
+    async _onBulkAssign(tasks) {
+        const targets = tasks;
+        const args = targets.map(([observer, task]) => ({
+            element: observer.element,
+            metadata: observer.metadata,
+            value: task.data.value
+        }));
+        const responses = await this._setValues(args);
+        this._applyValues(targets, responses, false);
+    }
+
+    _onElementChange(observer) {
+        const value = this._getElementValue(observer.element);
+        observer.value = value;
+        observer.hasValue = true;
+        this._assignTasks.enqueue(observer, {value});
+    }
+
+    _applyValues(targets, response, ignoreStale) {
+        if (!Array.isArray(response)) { return; }
+
+        for (let i = 0, ii = targets.length; i < ii; ++i) {
+            const [observer, task] = targets[i];
+            const {error, result} = response[i];
+            const stale = (task !== null && task.stale);
+
+            if (error) {
+                if (typeof this._onError === 'function') {
+                    this._onError(error, stale, observer.element, observer.metadata);
+                }
+                continue;
+            }
+
+            if (stale && !ignoreStale) { continue; }
+
+            observer.value = result;
+            observer.hasValue = true;
+            this._setElementValue(observer.element, result);
+        }
+    }
+
+    _createObserver(element) {
+        const metadata = this._createElementMetadata(element);
+        const nodeName = element.nodeName.toUpperCase();
+        const observer = {
+            element,
+            type: (nodeName === 'INPUT' ? element.type : null),
+            value: null,
+            hasValue: false,
+            onChange: null,
+            metadata
+        };
+        observer.onChange = this._onElementChange.bind(this, observer);
+
+        element.addEventListener('change', observer.onChange, false);
+
+        this._updateTasks.enqueue(observer);
+
+        return observer;
+    }
+
+    _removeObserver(element, observer) {
+        element.removeEventListener('change', observer.onChange, false);
+        observer.onChange = null;
+    }
+
+    _onObserverChildrenUpdated(element, observer) {
+        if (observer.hasValue) {
+            this._setElementValue(element, observer.value);
+        }
+    }
+
+    _isObserverStale(element, observer) {
+        const {type, metadata} = observer;
+        const nodeName = element.nodeName.toUpperCase();
+        return !(
+            type === (nodeName === 'INPUT' ? element.type : null) &&
+            this._compareElementMetadata(metadata, this._createElementMetadata(element))
+        );
+    }
+
+    _setElementValue(element, value) {
+        switch (element.nodeName.toUpperCase()) {
+            case 'INPUT':
+                switch (element.type) {
+                    case 'checkbox':
+                        element.checked = value;
+                        break;
+                    case 'text':
+                    case 'number':
+                        element.value = value;
+                        break;
+                }
+                break;
+            case 'TEXTAREA':
+            case 'SELECT':
+                element.value = value;
+                break;
+        }
+
+        const event = new CustomEvent('settingChanged', {detail: {value}});
+        element.dispatchEvent(event);
+    }
+
+    _getElementValue(element) {
+        switch (element.nodeName.toUpperCase()) {
+            case 'INPUT':
+                switch (element.type) {
+                    case 'checkbox':
+                        return !!element.checked;
+                    case 'text':
+                        return `${element.value}`;
+                    case 'number':
+                        return DOMDataBinder.convertToNumber(element.value, element);
+                }
+                break;
+            case 'TEXTAREA':
+            case 'SELECT':
+                return element.value;
+        }
+        return null;
+    }
+
+    // Utilities
+
+    static convertToNumber(value, constraints) {
+        value = parseFloat(value);
+        if (!Number.isFinite(value)) { return 0; }
+
+        let {min, max, step} = constraints;
+        min = DOMDataBinder.convertToNumberOrNull(min);
+        max = DOMDataBinder.convertToNumberOrNull(max);
+        step = DOMDataBinder.convertToNumberOrNull(step);
+        if (typeof min === 'number') { value = Math.max(value, min); }
+        if (typeof max === 'number') { value = Math.min(value, max); }
+        if (typeof step === 'number' && step !== 0) { value = Math.round(value / step) * step; }
+        return value;
+    }
+
+    static convertToNumberOrNull(value) {
+        if (typeof value !== 'number') {
+            if (typeof value !== 'string' || value.length === 0) {
+                return null;
+            }
+            value = parseFloat(value);
+        }
+        return !Number.isNaN(value) ? value : null;
+    }
+}
diff --git a/ext/js/dom/html-template-collection.js b/ext/js/dom/html-template-collection.js
new file mode 100644
index 00000000..52d5f3b0
--- /dev/null
+++ b/ext/js/dom/html-template-collection.js
@@ -0,0 +1,78 @@
+/*
+ * Copyright (C) 2020-2021  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+class HtmlTemplateCollection {
+    constructor(source) {
+        this._templates = new Map();
+
+        const sourceNode = (
+            typeof source === 'string' ?
+            new DOMParser().parseFromString(source, 'text/html') :
+            source
+        );
+
+        const pattern = /^([\w\W]+)-template$/;
+        for (const template of sourceNode.querySelectorAll('template')) {
+            const match = pattern.exec(template.id);
+            if (match === null) { continue; }
+            this._prepareTemplate(template);
+            this._templates.set(match[1], template);
+        }
+    }
+
+    instantiate(name) {
+        const template = this._templates.get(name);
+        return document.importNode(template.content.firstChild, true);
+    }
+
+    instantiateFragment(name) {
+        const template = this._templates.get(name);
+        return document.importNode(template.content, true);
+    }
+
+    getAllTemplates() {
+        return this._templates.values();
+    }
+
+    // Private
+
+    _prepareTemplate(template) {
+        if (template.dataset.removeWhitespaceText === 'true') {
+            this._removeWhitespaceText(template);
+        }
+    }
+
+    _removeWhitespaceText(template) {
+        const {content} = template;
+        const {TEXT_NODE} = Node;
+        const iterator = document.createNodeIterator(content, NodeFilter.SHOW_TEXT);
+        const removeNodes = [];
+        while (true) {
+            const node = iterator.nextNode();
+            if (node === null) { break; }
+            if (node.nodeType === TEXT_NODE && node.nodeValue.trim().length === 0) {
+                removeNodes.push(node);
+            }
+        }
+        for (const node of removeNodes) {
+            const {parentNode} = node;
+            if (parentNode !== null) {
+                parentNode.removeChild(node);
+            }
+        }
+    }
+}
diff --git a/ext/js/dom/panel-element.js b/ext/js/dom/panel-element.js
new file mode 100644
index 00000000..1ef61d6f
--- /dev/null
+++ b/ext/js/dom/panel-element.js
@@ -0,0 +1,103 @@
+/*
+ * Copyright (C) 2020-2021  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+class PanelElement extends EventDispatcher {
+    constructor({node, closingAnimationDuration}) {
+        super();
+        this._node = node;
+        this._closingAnimationDuration = closingAnimationDuration;
+        this._hiddenAnimatingClass = 'hidden-animating';
+        this._mutationObserver = null;
+        this._visible = false;
+        this._closeTimer = null;
+    }
+
+    get node() {
+        return this._node;
+    }
+
+    isVisible() {
+        return !this._node.hidden;
+    }
+
+    setVisible(value, animate=true) {
+        value = !!value;
+        if (this.isVisible() === value) { return; }
+
+        if (this._closeTimer !== null) {
+            clearTimeout(this._closeTimer);
+            this._completeClose(true);
+        }
+
+        const node = this._node;
+        const {classList} = node;
+        if (value) {
+            if (animate) { classList.add(this._hiddenAnimatingClass); }
+            getComputedStyle(node).getPropertyValue('display'); // Force update of CSS display property, allowing animation
+            classList.remove(this._hiddenAnimatingClass);
+            node.hidden = false;
+            node.focus();
+        } else {
+            if (animate) { classList.add(this._hiddenAnimatingClass); }
+            node.hidden = true;
+            if (animate) {
+                this._closeTimer = setTimeout(() => this._completeClose(false), this._closingAnimationDuration);
+            }
+        }
+    }
+
+    on(eventName, callback) {
+        if (eventName === 'visibilityChanged') {
+            if (this._mutationObserver === null) {
+                this._visible = this.isVisible();
+                this._mutationObserver = new MutationObserver(this._onMutation.bind(this));
+                this._mutationObserver.observe(this._node, {
+                    attributes: true,
+                    attributeFilter: ['hidden'],
+                    attributeOldValue: true
+                });
+            }
+        }
+        return super.on(eventName, callback);
+    }
+
+    off(eventName, callback) {
+        const result = super.off(eventName, callback);
+        if (eventName === 'visibilityChanged' && !this.hasListeners(eventName)) {
+            if (this._mutationObserver !== null) {
+                this._mutationObserver.disconnect();
+                this._mutationObserver = null;
+            }
+        }
+        return result;
+    }
+
+    // Private
+
+    _onMutation() {
+        const visible = this.isVisible();
+        if (this._visible === visible) { return; }
+        this._visible = visible;
+        this.trigger('visibilityChanged', {visible});
+    }
+
+    _completeClose(reopening) {
+        this._closeTimer = null;
+        this._node.classList.remove(this._hiddenAnimatingClass);
+        this.trigger('closeCompleted', {reopening});
+    }
+}
diff --git a/ext/js/dom/popup-menu.js b/ext/js/dom/popup-menu.js
new file mode 100644
index 00000000..9ad4e260
--- /dev/null
+++ b/ext/js/dom/popup-menu.js
@@ -0,0 +1,206 @@
+/*
+ * Copyright (C) 2020-2021  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+class PopupMenu extends EventDispatcher {
+    constructor(sourceElement, containerNode) {
+        super();
+        this._sourceElement = sourceElement;
+        this._containerNode = containerNode;
+        this._node = containerNode.querySelector('.popup-menu');
+        this._bodyNode = containerNode.querySelector('.popup-menu-body');
+        this._isClosed = false;
+        this._eventListeners = new EventListenerCollection();
+    }
+
+    get sourceElement() {
+        return this._sourceElement;
+    }
+
+    get containerNode() {
+        return this._containerNode;
+    }
+
+    get node() {
+        return this._node;
+    }
+
+    get bodyNode() {
+        return this._bodyNode;
+    }
+
+    get isClosed() {
+        return this._isClosed;
+    }
+
+    prepare() {
+        const items = this._bodyNode.querySelectorAll('.popup-menu-item');
+        this._setPosition();
+        this._containerNode.focus();
+
+        this._eventListeners.addEventListener(window, 'resize', this._onWindowResize.bind(this), false);
+        this._eventListeners.addEventListener(this._containerNode, 'click', this._onMenuContainerClick.bind(this), false);
+
+        const onMenuItemClick = this._onMenuItemClick.bind(this);
+        for (const item of items) {
+            this._eventListeners.addEventListener(item, 'click', onMenuItemClick, false);
+        }
+
+        PopupMenu.openMenus.add(this);
+
+        this._sourceElement.dispatchEvent(new CustomEvent('menuOpen', {
+            bubbles: false,
+            cancelable: false,
+            detail: {menu: this}
+        }));
+    }
+
+    close(cancelable=true) {
+        return this._close(null, 'close', cancelable);
+    }
+
+    // Private
+
+    _onMenuContainerClick(e) {
+        if (e.currentTarget !== e.target) { return; }
+        e.stopPropagation();
+        e.preventDefault();
+        this._close(null, 'outside', true);
+    }
+
+    _onMenuItemClick(e) {
+        const item = e.currentTarget;
+        if (item.disabled) { return; }
+        e.stopPropagation();
+        e.preventDefault();
+        this._close(item, 'item', true);
+    }
+
+    _onWindowResize() {
+        this._close(null, 'resize', true);
+    }
+
+    _setPosition() {
+        // Get flags
+        let horizontal = 1;
+        let vertical = 1;
+        let horizontalCover = 1;
+        let verticalCover = 1;
+        const positionInfo = this._sourceElement.dataset.menuPosition;
+        if (typeof positionInfo === 'string') {
+            const positionInfoSet = new Set(positionInfo.split(' '));
+
+            if (positionInfoSet.has('left')) {
+                horizontal = -1;
+            } else if (positionInfoSet.has('right')) {
+                horizontal = 1;
+            } else if (positionInfoSet.has('h-center')) {
+                horizontal = 0;
+            }
+
+            if (positionInfoSet.has('above')) {
+                vertical = -1;
+            } else if (positionInfoSet.has('below')) {
+                vertical = 1;
+            } else if (positionInfoSet.has('v-center')) {
+                vertical = 0;
+            }
+
+            if (positionInfoSet.has('cover')) {
+                horizontalCover = 1;
+                verticalCover = 1;
+            } else if (positionInfoSet.has('no-cover')) {
+                horizontalCover = -1;
+                verticalCover = -1;
+            }
+
+            if (positionInfoSet.has('h-cover')) {
+                horizontalCover = 1;
+            } else if (positionInfoSet.has('no-h-cover')) {
+                horizontalCover = -1;
+            }
+
+            if (positionInfoSet.has('v-cover')) {
+                verticalCover = 1;
+            } else if (positionInfoSet.has('no-v-cover')) {
+                verticalCover = -1;
+            }
+        }
+
+        // Position
+        const menu = this._node;
+        const fullRect = this._containerNode.getBoundingClientRect();
+        const sourceRect = this._sourceElement.getBoundingClientRect();
+        const menuRect = menu.getBoundingClientRect();
+        let top = menuRect.top;
+        let bottom = menuRect.bottom;
+        if (verticalCover === 1) {
+            const bodyRect = this._bodyNode.getBoundingClientRect();
+            top = bodyRect.top;
+            bottom = bodyRect.bottom;
+        }
+
+        let x = (
+            sourceRect.left +
+            sourceRect.width * ((-horizontal * horizontalCover + 1) * 0.5) +
+            menuRect.width * ((-horizontal + 1) * -0.5)
+        );
+        let y = (
+            sourceRect.top +
+            (menuRect.top - top) +
+            sourceRect.height * ((-vertical * verticalCover + 1) * 0.5) +
+            (bottom - top) * ((-vertical + 1) * -0.5)
+        );
+
+        x = Math.max(0.0, Math.min(fullRect.width - menuRect.width, x));
+        y = Math.max(0.0, Math.min(fullRect.height - menuRect.height, y));
+
+        menu.style.left = `${x}px`;
+        menu.style.top = `${y}px`;
+    }
+
+    _close(item, cause, cancelable) {
+        if (this._isClosed) { return true; }
+        const action = (item !== null ? item.dataset.menuAction : null);
+
+        const detail = {
+            menu: this,
+            item,
+            action,
+            cause
+        };
+        const result = this._sourceElement.dispatchEvent(new CustomEvent('menuClose', {bubbles: false, cancelable, detail}));
+        if (cancelable && !result) { return false; }
+
+        PopupMenu.openMenus.delete(this);
+
+        this._isClosed = true;
+        this._eventListeners.removeAllEventListeners();
+        if (this._containerNode.parentNode !== null) {
+            this._containerNode.parentNode.removeChild(this._containerNode);
+        }
+
+        this.trigger('close', detail);
+        return true;
+    }
+}
+
+Object.defineProperty(PopupMenu, 'openMenus', {
+    configurable: false,
+    enumerable: true,
+    writable: false,
+    value: new Set()
+});
diff --git a/ext/js/dom/selector-observer.js b/ext/js/dom/selector-observer.js
new file mode 100644
index 00000000..2f3fa49e
--- /dev/null
+++ b/ext/js/dom/selector-observer.js
@@ -0,0 +1,255 @@
+/*
+ * Copyright (C) 2020-2021  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/**
+ * Class which is used to observe elements matching a selector in specific element.
+ */
+class SelectorObserver {
+    /**
+     * Creates a new instance.
+     * @param selector A string CSS selector used to find elements.
+     * @param ignoreSelector A string CSS selector used to filter elements, or null for no filtering.
+     * @param onAdded A function which is invoked for each element that is added that matches the selector.
+     *   The signature is (element) => data.
+     * @param onRemoved A function which is invoked for each element that is removed, or null.
+     *   The signature is (element, data) => void.
+     * @param onChildrenUpdated A function which is invoked for each element which has its children updated, or null.
+     *   The signature is (element, data) => void.
+     * @param isStale A function which checks if the data is stale for a given element, or null.
+     *   If the element is stale, it will be removed and potentially re-added.
+     *   The signature is (element, data) => bool.
+     */
+    constructor({selector, ignoreSelector=null, onAdded=null, onRemoved=null, onChildrenUpdated=null, isStale=null}) {
+        this._selector = selector;
+        this._ignoreSelector = ignoreSelector;
+        this._onAdded = onAdded;
+        this._onRemoved = onRemoved;
+        this._onChildrenUpdated = onChildrenUpdated;
+        this._isStale = isStale;
+        this._observingElement = null;
+        this._mutationObserver = new MutationObserver(this._onMutation.bind(this));
+        this._elementMap = new Map(); // Map([element => observer]...)
+        this._elementAncestorMap = new Map(); // Map([element => Set([observer]...)]...)
+        this._isObserving = false;
+    }
+
+    /**
+     * Returns whether or not an element is currently being observed.
+     * @returns True if an element is being observed, false otherwise.
+     */
+    get isObserving() {
+        return this._observingElement !== null;
+    }
+
+    /**
+     * Starts DOM mutation observing the target element.
+     * @param element The element to observe changes in.
+     * @param attributes A boolean for whether or not attribute changes should be observed.
+     * @throws An error if element is null.
+     * @throws An error if an element is already being observed.
+     */
+    observe(element, attributes=false) {
+        if (element === null) {
+            throw new Error('Invalid element');
+        }
+        if (this.isObserving) {
+            throw new Error('Instance is already observing an element');
+        }
+
+        this._observingElement = element;
+        this._mutationObserver.observe(element, {
+            attributes: !!attributes,
+            childList: true,
+            subtree: true
+        });
+
+        this._onMutation([{
+            type: 'childList',
+            target: element.parentNode,
+            addedNodes: [element],
+            removedNodes: []
+        }]);
+    }
+
+    /**
+     * Stops observing the target element.
+     */
+    disconnect() {
+        if (!this.isObserving) { return; }
+
+        this._mutationObserver.disconnect();
+        this._observingElement = null;
+
+        for (const observer of this._elementMap.values()) {
+            this._removeObserver(observer);
+        }
+    }
+
+    /**
+     * Returns an iterable list of [element, data] pairs.
+     * @yields A sequence of [element, data] pairs.
+     */
+    *entries() {
+        for (const [element, {data}] of this._elementMap) {
+            yield [element, data];
+        }
+    }
+
+    /**
+     * Returns an iterable list of data for every element.
+     * @yields A sequence of data values.
+     */
+    *datas() {
+        for (const {data} of this._elementMap.values()) {
+            yield data;
+        }
+    }
+
+    // Private
+
+    _onMutation(mutationList) {
+        for (const mutation of mutationList) {
+            switch (mutation.type) {
+                case 'childList':
+                    this._onChildListMutation(mutation);
+                    break;
+                case 'attributes':
+                    this._onAttributeMutation(mutation);
+                    break;
+            }
+        }
+    }
+
+    _onChildListMutation({addedNodes, removedNodes, target}) {
+        const selector = this._selector;
+        const ELEMENT_NODE = Node.ELEMENT_NODE;
+
+        for (const node of removedNodes) {
+            const observers = this._elementAncestorMap.get(node);
+            if (typeof observers === 'undefined') { continue; }
+            for (const observer of observers) {
+                this._removeObserver(observer);
+            }
+        }
+
+        for (const node of addedNodes) {
+            if (node.nodeType !== ELEMENT_NODE) { continue; }
+            if (node.matches(selector)) {
+                this._createObserver(node);
+            }
+            for (const childNode of node.querySelectorAll(selector)) {
+                this._createObserver(childNode);
+            }
+        }
+
+        if (
+            this._onChildrenUpdated !== null &&
+            (addedNodes.length !== 0 || addedNodes.length !== 0)
+        ) {
+            for (let node = target; node !== null; node = node.parentNode) {
+                const observer = this._elementMap.get(node);
+                if (typeof observer !== 'undefined') {
+                    this._onObserverChildrenUpdated(observer);
+                }
+            }
+        }
+    }
+
+    _onAttributeMutation({target}) {
+        const selector = this._selector;
+        const observers = this._elementAncestorMap.get(target);
+        if (typeof observers !== 'undefined') {
+            for (const observer of observers) {
+                const element = observer.element;
+                if (
+                    !element.matches(selector) ||
+                    this._shouldIgnoreElement(element) ||
+                    this._isObserverStale(observer)
+                ) {
+                    this._removeObserver(observer);
+                }
+            }
+        }
+
+        if (target.matches(selector)) {
+            this._createObserver(target);
+        }
+    }
+
+    _createObserver(element) {
+        if (this._elementMap.has(element) || this._shouldIgnoreElement(element) || this._onAdded === null) { return; }
+
+        const data = this._onAdded(element);
+        const ancestors = this._getAncestors(element);
+        const observer = {element, ancestors, data};
+
+        this._elementMap.set(element, observer);
+
+        for (const ancestor of ancestors) {
+            let observers = this._elementAncestorMap.get(ancestor);
+            if (typeof observers === 'undefined') {
+                observers = new Set();
+                this._elementAncestorMap.set(ancestor, observers);
+            }
+            observers.add(observer);
+        }
+    }
+
+    _removeObserver(observer) {
+        const {element, ancestors, data} = observer;
+
+        this._elementMap.delete(element);
+
+        for (const ancestor of ancestors) {
+            const observers = this._elementAncestorMap.get(ancestor);
+            if (typeof observers === 'undefined') { continue; }
+
+            observers.delete(observer);
+            if (observers.size === 0) {
+                this._elementAncestorMap.delete(ancestor);
+            }
+        }
+
+        if (this._onRemoved !== null) {
+            this._onRemoved(element, data);
+        }
+    }
+
+    _onObserverChildrenUpdated(observer) {
+        this._onChildrenUpdated(observer.element, observer.data);
+    }
+
+    _isObserverStale(observer) {
+        return (this._isStale !== null && this._isStale(observer.element, observer.data));
+    }
+
+    _shouldIgnoreElement(element) {
+        return (this._ignoreSelector !== null && element.matches(this._ignoreSelector));
+    }
+
+    _getAncestors(node) {
+        const root = this._observingElement;
+        const results = [];
+        while (true) {
+            results.push(node);
+            if (node === root) { break; }
+            node = node.parentNode;
+            if (node === null) { break; }
+        }
+        return results;
+    }
+}
diff --git a/ext/js/dom/window-scroll.js b/ext/js/dom/window-scroll.js
new file mode 100644
index 00000000..33577795
--- /dev/null
+++ b/ext/js/dom/window-scroll.js
@@ -0,0 +1,110 @@
+/*
+ * Copyright (C) 2019-2021  Yomichan Authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+class WindowScroll {
+    constructor(node) {
+        this._node = node;
+        this._animationRequestId = null;
+        this._animationStartTime = 0;
+        this._animationStartX = 0;
+        this._animationStartY = 0;
+        this._animationEndTime = 0;
+        this._animationEndX = 0;
+        this._animationEndY = 0;
+        this._requestAnimationFrameCallback = this._onAnimationFrame.bind(this);
+    }
+
+    get x() {
+        return this._node !== null ? this._node.scrollLeft : window.scrollX || window.pageXOffset;
+    }
+
+    get y() {
+        return this._node !== null ? this._node.scrollTop : window.scrollY || window.pageYOffset;
+    }
+
+    toY(y) {
+        this.to(this.x, y);
+    }
+
+    toX(x) {
+        this.to(x, this.y);
+    }
+
+    to(x, y) {
+        this.stop();
+        this._scroll(x, y);
+    }
+
+    animate(x, y, time) {
+        this._animationStartX = this.x;
+        this._animationStartY = this.y;
+        this._animationStartTime = window.performance.now();
+        this._animationEndX = x;
+        this._animationEndY = y;
+        this._animationEndTime = this._animationStartTime + time;
+        this._animationRequestId = window.requestAnimationFrame(this._requestAnimationFrameCallback);
+    }
+
+    stop() {
+        if (this._animationRequestId === null) {
+            return;
+        }
+
+        window.cancelAnimationFrame(this._animationRequestId);
+        this._animationRequestId = null;
+    }
+
+    // Private
+
+    _onAnimationFrame(time) {
+        if (time >= this._animationEndTime) {
+            this._scroll(this._animationEndX, this._animationEndY);
+            this._animationRequestId = null;
+            return;
+        }
+
+        const t = this._easeInOutCubic((time - this._animationStartTime) / (this._animationEndTime - this._animationStartTime));
+        this._scroll(
+            this._lerp(this._animationStartX, this._animationEndX, t),
+            this._lerp(this._animationStartY, this._animationEndY, t)
+        );
+
+        this._animationRequestId = window.requestAnimationFrame(this._requestAnimationFrameCallback);
+    }
+
+    _easeInOutCubic(t) {
+        if (t < 0.5) {
+            return (4.0 * t * t * t);
+        } else {
+            t = 1.0 - t;
+            return 1.0 - (4.0 * t * t * t);
+        }
+    }
+
+    _lerp(start, end, percent) {
+        return (end - start) * percent + start;
+    }
+
+    _scroll(x, y) {
+        if (this._node !== null) {
+            this._node.scrollLeft = x;
+            this._node.scrollTop = y;
+        } else {
+            window.scroll(x, y);
+        }
+    }
+}