document.js refactor (#719)

* Refactor document.js into a class

* Move public functions first

* Rename private functions

* Rename

* Rename argument

* Use instance of DocumentUtil

* Update tests

* Refactor

* Rename (test-)document.js to (test-)document-util.js

4 files changed, 387 insertions, 378 deletions

diff --git a/ext/fg/float.html b/ext/fg/float.html
index 17378713..51aa2350 100644
--- a/ext/fg/float.html
+++ b/ext/fg/float.html
@@ -51,7 +51,7 @@
         <script src="/mixed/js/api.js"></script>
         <script src="/mixed/js/japanese.js"></script>
 
-        <script src="/fg/js/document.js"></script>
+        <script src="/fg/js/document-util.js"></script>
         <script src="/fg/js/dom-text-scanner.js"></script>
         <script src="/fg/js/source.js"></script>
         <script src="/mixed/js/audio-system.js"></script>
diff --git a/ext/fg/js/document-util.js b/ext/fg/js/document-util.js
new file mode 100644
index 00000000..d3bba30f
--- /dev/null
+++ b/ext/fg/js/document-util.js
@@ -0,0 +1,381 @@
+/*
+ * 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);
+    }
+}
diff --git a/ext/fg/js/document.js b/ext/fg/js/document.js
deleted file mode 100644
index c288502c..00000000
--- a/ext/fg/js/document.js
+++ /dev/null
@@ -1,374 +0,0 @@
-/*
- * 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
- */
-
-const REGEX_TRANSPARENT_COLOR = /rgba\s*\([^)]*,\s*0(?:\.0+)?\s*\)/;
-
-function docSetImposterStyle(style, propertyName, value) {
-    style.setProperty(propertyName, value, 'important');
-}
-
-function docImposterCreate(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;
-    docSetImposterStyle(containerStyle, 'all', 'initial');
-    docSetImposterStyle(containerStyle, 'position', 'absolute');
-    docSetImposterStyle(containerStyle, 'left', '0');
-    docSetImposterStyle(containerStyle, 'top', '0');
-    docSetImposterStyle(containerStyle, 'width', `${documentRect.width}px`);
-    docSetImposterStyle(containerStyle, 'height', `${documentRect.height}px`);
-    docSetImposterStyle(containerStyle, 'overflow', 'hidden');
-    docSetImposterStyle(containerStyle, 'opacity', '0');
-
-    docSetImposterStyle(containerStyle, 'pointer-events', 'none');
-    docSetImposterStyle(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];
-        docSetImposterStyle(imposterStyle, property, elementStyle.getPropertyValue(property));
-    }
-    docSetImposterStyle(imposterStyle, 'position', 'absolute');
-    docSetImposterStyle(imposterStyle, 'top', `${top}px`);
-    docSetImposterStyle(imposterStyle, 'left', `${left}px`);
-    docSetImposterStyle(imposterStyle, 'margin', '0');
-    docSetImposterStyle(imposterStyle, 'pointer-events', 'auto');
-
-    if (isTextarea) {
-        if (elementStyle.overflow === 'visible') {
-            docSetImposterStyle(imposterStyle, 'overflow', 'auto');
-        }
-    } else {
-        docSetImposterStyle(imposterStyle, 'overflow', 'hidden');
-        docSetImposterStyle(imposterStyle, 'white-space', 'nowrap');
-        docSetImposterStyle(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);
-        docSetImposterStyle(imposterStyle, 'width', `${width}px`);
-        docSetImposterStyle(imposterStyle, 'height', `${height}px`);
-    }
-    if (imposterRect.x !== elementRect.x || imposterRect.y !== elementRect.y) {
-        left += (elementRect.left - imposterRect.left);
-        top += (elementRect.top - imposterRect.top);
-        docSetImposterStyle(imposterStyle, 'left', `${left}px`);
-        docSetImposterStyle(imposterStyle, 'top', `${top}px`);
-    }
-
-    imposter.scrollTop = element.scrollTop;
-    imposter.scrollLeft = element.scrollLeft;
-
-    return [imposter, container];
-}
-
-function docElementsFromPoint(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] : [];
-}
-
-function docRangeFromPoint(x, y, deepDomScan) {
-    const elements = docElementsFromPoint(x, y, deepDomScan);
-    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] = docImposterCreate(element, false);
-                break;
-            case 'TEXTAREA':
-                imposterSourceElement = element;
-                [imposter, imposterContainer] = docImposterCreate(element, true);
-                break;
-        }
-    }
-
-    const range = caretRangeFromPointExt(x, y, deepDomScan ? elements : []);
-    if (range !== null) {
-        if (imposter !== null) {
-            docSetImposterStyle(imposterContainer.style, 'z-index', '-2147483646');
-            docSetImposterStyle(imposter.style, 'pointer-events', 'none');
-        }
-        return new TextSourceRange(range, '', imposterContainer, imposterSourceElement);
-    } else {
-        if (imposterContainer !== null) {
-            imposterContainer.parentNode.removeChild(imposterContainer);
-        }
-        return null;
-    }
-}
-
-function docSentenceExtract(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
-    };
-}
-
-function 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 (!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 (!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;
-}
-
-function isWhitespace(string) {
-    return string.trim().length === 0;
-}
-
-const caretRangeFromPoint = (() => {
-    if (typeof document.caretRangeFromPoint === 'function') {
-        // Chrome, Edge
-        return (x, y) => document.caretRangeFromPoint(x, y);
-    }
-
-    if (typeof document.caretPositionFromPoint === 'function') {
-        // Firefox
-        return (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;
-        };
-    }
-
-    // No support
-    return () => null;
-})();
-
-function caretRangeFromPointExt(x, y, elements) {
-    const modifications = [];
-    try {
-        let i = 0;
-        let startContinerPre = null;
-        while (true) {
-            const range = caretRangeFromPoint(x, y);
-            if (range === null) {
-                return null;
-            }
-
-            const startContainer = range.startContainer;
-            if (startContinerPre !== startContainer) {
-                if (isPointInRange(x, y, range)) {
-                    return range;
-                }
-                startContinerPre = startContainer;
-            }
-
-            i = disableTransparentElement(elements, i, modifications);
-            if (i < 0) {
-                return null;
-            }
-        }
-    } finally {
-        if (modifications.length > 0) {
-            restoreElementStyleModifications(modifications);
-        }
-    }
-}
-
-function disableTransparentElement(elements, i, modifications) {
-    while (true) {
-        if (i >= elements.length) {
-            return -1;
-        }
-
-        const element = elements[i++];
-        if (isElementTransparent(element)) {
-            const style = element.hasAttribute('style') ? element.getAttribute('style') : null;
-            modifications.push({element, style});
-            element.style.setProperty('pointer-events', 'none', 'important');
-            return i;
-        }
-    }
-}
-
-function restoreElementStyleModifications(modifications) {
-    for (const {element, style} of modifications) {
-        if (style === null) {
-            element.removeAttribute('style');
-        } else {
-            element.setAttribute('style', style);
-        }
-    }
-}
-
-function 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' && isColorTransparent(style.backgroundColor))
-    );
-}
-
-function isColorTransparent(cssColor) {
-    return REGEX_TRANSPARENT_COLOR.test(cssColor);
-}
diff --git a/ext/fg/js/frontend.js b/ext/fg/js/frontend.js
index 1214e74b..ccffbab6 100644
--- a/ext/fg/js/frontend.js
+++ b/ext/fg/js/frontend.js
@@ -17,12 +17,12 @@
 
 /* global
  * DOM
+ * DocumentUtil
  * FrameOffsetForwarder
  * PopupProxy
  * TextScanner
  * TextSourceElement
  * api
- * docSentenceExtract
  */
 
 class Frontend {
@@ -36,11 +36,13 @@ class Frontend {
         this._lastShowPromise = Promise.resolve();
         this._activeModifiers = new Set();
         this._optionsUpdatePending = false;
+        this._documentUtil = new DocumentUtil();
         this._textScanner = new TextScanner({
             node: window,
             ignoreElements: this._ignoreElements.bind(this),
             ignorePoint: this._ignorePoint.bind(this),
-            search: this._search.bind(this)
+            search: this._search.bind(this),
+            documentUtil: this._documentUtil
         });
 
         const {
@@ -432,7 +434,7 @@ class Frontend {
         const {url} = optionsContext;
         const sentenceExtent = this._options.anki.sentenceExt;
         const layoutAwareScan = this._options.scanning.layoutAwareScan;
-        const sentence = docSentenceExtract(textSource, sentenceExtent, layoutAwareScan);
+        const sentence = this._documentUtil.extractSentence(textSource, sentenceExtent, layoutAwareScan);
         const query = textSource.text();
         const details = {
             focus,