Add option to display tooltip on link hover

src/BasePlatform.ts

@@ -222,6 +222,14 @@ export default abstract class BasePlatform {
         }
     }
 
+    /**
+     * Returns true if the platform requires URL previews in tooltips, otherwise false.
+     * @returns {boolean} whether the platform requires URL previews in tooltips
+     */
+    public needsUrlTooltips(): boolean {
+        return false;
+    }
+
     /**
      * Returns a promise that resolves to a string representing the current version of the application.
      */

src/HtmlUtils.tsx

@@ -18,6 +18,7 @@ limitations under the License.
 */
 
 import React, { ReactNode } from 'react';
+import ReactDOM from 'react-dom';
 import sanitizeHtml from 'sanitize-html';
 import cheerio from 'cheerio';
 import classNames from 'classnames';
@@ -35,6 +36,8 @@ import { getEmojiFromUnicode } from "./emoji";
 import { mediaFromMxc } from "./customisations/Media";
 import { ELEMENT_URL_PATTERN, options as linkifyMatrixOptions } from './linkify-matrix';
 import { stripHTMLReply, stripPlainReply } from './utils/Reply';
+import TextWithTooltip from './components/views/elements/TextWithTooltip';
+import PlatformPeg from './PlatformPeg';
 
 // Anything outside the basic multilingual plane will be a surrogate pair
 const SURROGATE_PAIR_PATTERN = /([\ud800-\udbff])([\udc00-\udfff])/;
@@ -635,6 +638,66 @@ export function linkifyAndSanitizeHtml(dirtyHtml: string, options = linkifyMatri
     return sanitizeHtml(linkifyString(dirtyHtml, options), sanitizeHtmlParams);
 }
 
+/**
+ * Turns a given URL into an absolute one.
+ * @param {string} url The URL transform
+ * @returns {string} Absolute URL
+ */
+const getAbsoluteUrl = (() => {
+    // Use a single cached anchor element stored in a closure to avoid having to recreate
+    // a new one on every call and increase performance
+    let a: HTMLAnchorElement;
+
+    return (url: string) => {
+        if (!a) {
+            a = document.createElement('a');
+        }
+        a.href = url;
+        return a.href;
+    };
+})();
+
+/**
+ * Recurses depth-first through a DOM tree, adding tooltip previews for link elements.
+ *
+ * @param {Element[]} rootNodes - a list of sibling DOM nodes to traverse to try
+ *   to add tooltips.
+ * @param {Element[]} ignoredNodes: a list of nodes to not recurse into.
+ */
+export function tooltipifyLinks(rootNodes: ArrayLike<Element>, ignoredNodes: Element[]) {
+    if (!PlatformPeg.get()?.needsUrlTooltips()) {
+        return;
+    }
+
+    let node = rootNodes[0];
+
+    while (node) {
+        let tooltipified = false;
+
+        if (ignoredNodes.indexOf(node) >= 0) {
+            node = node.nextSibling as Element;
+            continue;
+        }
+
+        if (node.tagName === "A" && node.getAttribute("href")
+            && node.getAttribute("href") !== node.textContent.trim()
+        ) {
+            const href = node.getAttribute("href");
+            const tooltip = <TextWithTooltip tooltip={getAbsoluteUrl(href)}>
+                <span dangerouslySetInnerHTML={{ __html: node.innerHTML }} />
+            </TextWithTooltip>;
+            ReactDOM.render(tooltip, node);
+            tooltipified = true;
+        }
+
+        if (node.childNodes && node.childNodes.length && !tooltipified) {
+            tooltipifyLinks(node.childNodes as NodeListOf<Element>, ignoredNodes);
+        }
+
+        node = node.nextSibling as Element;
+    }
+}
+
 /**
  * Returns if a node is a block element or not.
  * Only takes html nodes into account that are allowed in matrix messages.

src/components/views/messages/EditHistoryMessage.tsx

@@ -93,8 +93,16 @@ export default class EditHistoryMessage extends React.PureComponent<IProps, ISta
         }
     }
 
+    private tooltipifyLinks(): void {
+        // not present for redacted events
+        if (this.content.current) {
+            HtmlUtils.tooltipifyLinks(this.content.current.children, this.pills);
+        }
+    }
+
     public componentDidMount(): void {
         this.pillifyLinks();
+        this.tooltipifyLinks();
     }
 
     public componentWillUnmount(): void {
@@ -107,6 +115,7 @@ export default class EditHistoryMessage extends React.PureComponent<IProps, ISta
 
     public componentDidUpdate(): void {
         this.pillifyLinks();
+        this.tooltipifyLinks();
     }
 
     private renderActionBar(): JSX.Element {

src/components/views/messages/TextualBody.tsx

@@ -91,6 +91,7 @@ export default class TextualBody extends React.Component<IBodyProps, IState> {
         // we should be pillify them here by doing the linkifying BEFORE the pillifying.
         pillifyLinks([this.contentRef.current], this.props.mxEvent, this.pills);
         HtmlUtils.linkifyElement(this.contentRef.current);
+        HtmlUtils.tooltipifyLinks([this.contentRef.current], this.pills);
         this.calculateUrlPreview();
 
         if (this.props.mxEvent.getContent().format === "org.matrix.custom.html") {