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diff --git a/ext/fg/js/document-util.js b/ext/fg/js/document-util.js
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+/*
+ * Copyright (C) 2016-2020  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
+ * DOM
+ * 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;
+        }
+    }
+
+    extractSentence(source, extent, layoutAwareScan) {
+        const quotesFwd = {'「': '」', '『': '』', "'": "'", '"': '"'};
+        const quotesBwd = {'」': '「', '』': '『', "'": "'", '"': '"'};
+        const terminators = '…。．.？?！!';
+
+        const sourceLocal = source.clone();
+        const position = sourceLocal.setStartOffset(extent, layoutAwareScan);
+        sourceLocal.setEndOffset(extent * 2 - position, layoutAwareScan, true);
+        const content = sourceLocal.text();
+
+        let quoteStack = [];
+
+        let startPos = 0;
+        for (let i = position; i >= startPos; --i) {
+            const c = content[i];
+
+            if (c === '\n') {
+                startPos = i + 1;
+                break;
+            }
+
+            if (quoteStack.length === 0 && (terminators.includes(c) || c in quotesFwd)) {
+                startPos = i + 1;
+                break;
+            }
+
+            if (quoteStack.length > 0 && c === quoteStack[0]) {
+                quoteStack.pop();
+            } else if (c in quotesBwd) {
+                quoteStack.unshift(quotesBwd[c]);
+            }
+        }
+
+        quoteStack = [];
+
+        let endPos = content.length;
+        for (let i = position; i <= endPos; ++i) {
+            const c = content[i];
+
+            if (c === '\n') {
+                endPos = i + 1;
+                break;
+            }
+
+            if (quoteStack.length === 0) {
+                if (terminators.includes(c)) {
+                    endPos = i + 1;
+                    break;
+                } else if (c in quotesBwd) {
+                    endPos = i;
+                    break;
+                }
+            }
+
+            if (quoteStack.length > 0 && c === quoteStack[0]) {
+                quoteStack.pop();
+            } else if (c in quotesFwd) {
+                quoteStack.unshift(quotesFwd[c]);
+            }
+        }
+
+        const text = content.substring(startPos, endPos);
+        const padding = text.length - text.replace(/^\s+/, '').length;
+
+        return {
+            text: text.trim(),
+            offset: position - startPos - padding
+        };
+    }
+
+    // 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) && DOM.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) && DOM.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);
+    }
+}