Add LaTeX support

.eslintignore

@@ -1,2 +1,6 @@
 **/flow-typed/**
-**/__flow-tests__/**
+**/__flow-tests__/**
+
+# Third-party minified code. (Also, this is a staging directory, and is
+# untracked by git.)
+src/webview/static/katex

package.json

@@ -40,6 +40,7 @@
     "blueimp-md5": "^2.10.0",
     "color": "^3.0.0",
     "date-fns": "^1.29.0",
+    "katex": "^0.10.2",
     "lodash.escape": "^4.0.1",
     "lodash.isequal": "^4.4.0",
     "lodash.omit": "^4.5.0",

src/webview/css/css.js

@@ -1,11 +1,73 @@
 /* @flow strict-local */
+import { Platform } from 'react-native';
 import type { ThemeName } from '../../types';
 import cssPygments from './cssPygments';
 import cssEmojis from './cssEmojis';
 import cssNight from './cssNight';
 
+/** CSS fragment to support scrolling of KaTeX formulae. */
+/*
+  By default, KaTeX renders (non-inline) math into a div of fixed precomputed
+  width. This will be cut off by the edge of the screen when the formula is too
+  long -- and on a mobile device, that's very nearly always.
+
+  The naïve solution of simply giving some part of the KaTeX fragment itself an
+  `overflow-x: auto` style breaks terribly:
+    * Margin collapsing no longer works, causing rendering artifacts. (This is
+      particularly visible on integral signs.)
+    * `overflow-y: hidden` isn't respected. If KaTeX has used negative-position
+      struts in its rendering (which it does frequently), there will always be
+      vertical scrollability. (This may be a Chrome bug.)
+
+  Instead, we modify the provided DOM to wrap each `.katex-display` div with two
+  additional elements: the outer element is scrollable and `display: block`,
+  while the inner element is fixed and `display: inline-block`. This suffices to
+  insulate the KaTeX elements from the deleterious effects of scrollability.
+
+  The inner of these elements also receives a border, to act as a UI hint
+  indicating that scrolling is necessary: the right border will be cut off when
+  the rendered element is too large. (The KaTeX itself will also be truncated,
+  of course, but this may not be apparent if the cutoff falls between two
+  symbols.)
+
+  We also cut the KaTeX-provided margin somewhat. (Since the KaTeX fragment is
+  isolated in the new divs, margin-collapsing can no longer occur.)
+*/
+const katexScrollStyle = `<style id="katex-mobile-scroll">
+.zulip-katex-outer {
+  display: block;
+  overflow-x: auto;
+  text-align: center;
+}
+.zulip-katex-inner {
+  display: inline-block;
+  border: 1px solid hsla(187, 35%, 51%, .5);
+  border-radius: 3px;
+}
+/* adjust/override KaTeX-provided CSS */
+.zulip-katex-inner .katex-display { margin: 0.5em 0.25em; }
+</style>`;
+
+/**
+ * Fix KaTeX frac-line elements disappearing.
+ *
+ * This is a hack, but it's probably better than not having fraction lines on
+ * low-resolution phones. It's only known to be useful under Chrome and Android,
+ * so we only include it there.
+ *
+ * See, among others:
+ *   https://github.com/KaTeX/KaTeX/issues/824
+ *   https://github.com/KaTeX/KaTeX/issues/916
+ *   https://github.com/KaTeX/KaTeX/pull/1249
+ *   https://github.com/KaTeX/KaTeX/issues/1775
+ */
+const katexFraclineHackStyle = `<style id="katex-frac-line-hack">
+.katex .mfrac .frac-line { border-bottom-width: 1px !important; }
+</style>`;
+
 export default (theme: ThemeName) => `
 <link rel='stylesheet' type='text/css' href='./base.css'>
+<link rel='stylesheet' type='text/css' href='./katex/katex.min.css'>
 <style>
 ${theme === 'night' ? cssNight : ''}
 ${cssPygments(theme === 'night')}
@@ -16,5 +78,7 @@ ${cssEmojis}
   display: none;
 }
 </style>
+${katexScrollStyle}
+${Platform.OS === 'android' ? katexFraclineHackStyle : '<!-- Safari -->'}
 <style id="generated-styles"></style>
 `;

src/webview/js/fixup-katex.js

@@ -0,0 +1,35 @@
+/* @flow strict */
+
+// Auxiliary function, for brevity.
+const makeDiv = (className: string): Element => {
+  const element = document.createElement('div');
+  element.classList.add(className);
+  return element;
+};
+
+/**
+ * Adjust non-inline KaTeX equation element structure.
+ *
+ * This surrounds existing `.katex-display` elements (the KaTeX wrapper for
+ * non-inline equations) with shim `<div>`s to allow horizontal scrolling.
+ *
+ * (The actual functionality of these elements is provided by CSS rules.)
+ */
+const fixupKatex = (root: Element) => {
+  Array.from(root.querySelectorAll('.katex-display')).forEach(kd => {
+    // nodes returned by `querySelectorAll` will always have a valid parent node
+    const parent: Node = ((kd.parentNode: ?Node): $FlowFixMe);
+
+    // Nest each `.katex-display` element within two new <div> elements.
+    // Notionally:
+    //   s/ (.katex-display) / div.z-k-outer > div.z-k-inner > $1 /
+    const outer = makeDiv('zulip-katex-outer');
+    const inner = makeDiv('zulip-katex-inner');
+
+    outer.appendChild(inner);
+    parent.replaceChild(outer, kd);
+    inner.appendChild(kd);
+  });
+};
+
+export default fixupKatex;

src/webview/js/generatedEs3.js

@@ -37,8 +37,7 @@ var compiledWebviewJs = (function (exports) {
 
   var inlineApiRoutes = ['/user_uploads/', '/thumbnail?', '/avatar/'];
 
-  var rewriteImageUrls = function rewriteImageUrls(auth) {
-    var element = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : document;
+  var rewriteImageUrls = function rewriteImageUrls(auth, element) {
     var realm = new URL(auth.realm);
     var imageTags = [].concat(element instanceof HTMLImageElement ? [element] : [], Array.from(element.getElementsByTagName('img')));
     imageTags.forEach(function (img) {
@@ -71,6 +70,23 @@ var compiledWebviewJs = (function (exports) {
     });
   };
 
+  var makeDiv = function makeDiv(className) {
+    var element = document.createElement('div');
+    element.classList.add(className);
+    return element;
+  };
+
+  var fixupKatex = function fixupKatex(root) {
+    Array.from(root.querySelectorAll('.katex-display')).forEach(function (kd) {
+      var parent = kd.parentNode;
+      var outer = makeDiv('zulip-katex-outer');
+      var inner = makeDiv('zulip-katex-inner');
+      outer.appendChild(inner);
+      parent.replaceChild(outer, kd);
+      inner.appendChild(kd);
+    });
+  };
+
   if (!Array.from) {
     Array.from = function from(arr) {
       return Array.prototype.slice.call(arr);
@@ -404,7 +420,15 @@ var compiledWebviewJs = (function (exports) {
     window.scrollBy(0, newBoundRect.top - prevBoundTop);
   };
 
+  var processIncomingHtml = function processIncomingHtml(auth, root) {
+    rewriteImageUrls(auth, root);
+    fixupKatex(root);
+  };
+
   var handleUpdateEventContent = function handleUpdateEventContent(uevent) {
+    var contentNode = document.createElement('div');
+    contentNode.innerHTML = uevent.content;
+    processIncomingHtml(uevent.auth, contentNode);
     var target;
 
     if (uevent.updateStrategy === 'replace') {
@@ -424,8 +448,7 @@ var compiledWebviewJs = (function (exports) {
       target = findPreserveTarget();
     }
 
-    documentBody.innerHTML = uevent.content;
-    rewriteImageUrls(uevent.auth);
+    documentBody.innerHTML = contentNode.innerHTML;
 
     if (target.type === 'bottom') {
       scrollToBottom();
@@ -445,8 +468,8 @@ var compiledWebviewJs = (function (exports) {
       document.addEventListener('message', handleMessageEvent);
     }
 
+    processIncomingHtml(auth, documentBody);
     scrollToAnchor(anchor);
-    rewriteImageUrls(auth);
     sendScrollMessageIfListShort();
     scrollEventsDisabled = false;
   };

src/webview/js/js.js

@@ -12,6 +12,7 @@ import type {
 import type { MessageListEvent } from '../webViewEventHandlers';
 
 import rewriteImageUrls from './rewriteImageUrls';
+import fixupKatex from './fixup-katex';
 
 /*
  * Supported platforms:
@@ -476,7 +477,22 @@ const scrollToPreserve = (msgId: number, prevBoundTop: number) => {
   window.scrollBy(0, newBoundRect.top - prevBoundTop);
 };
 
+/**
+ * Fix up supplied HTML as needed for display.
+ *
+ * The root itself must not need fixups.
+ */
+const processIncomingHtml = (auth: Auth, root: Element) => {
+  rewriteImageUrls(auth, root);
+  fixupKatex(root);
+};
+
 const handleUpdateEventContent = (uevent: WebViewUpdateEventContent) => {
+  // Perform preprocessing on the webview content.
+  const contentNode: HTMLDivElement = document.createElement('div');
+  contentNode.innerHTML = uevent.content;
+  processIncomingHtml(uevent.auth, contentNode);
+
   let target: ScrollTarget;
   if (uevent.updateStrategy === 'replace') {
     target = { type: 'none' };
@@ -492,9 +508,11 @@ const handleUpdateEventContent = (uevent: WebViewUpdateEventContent) => {
     target = findPreserveTarget();
   }
 
-  documentBody.innerHTML = uevent.content;
-
-  rewriteImageUrls(uevent.auth);
+  // TODO: eliminate the needless reserialization and deserialization
+  // step here, via something along the lines of
+  //     documentBody.replaceChild(oldContentNode, contentNode)
+  // (which currently breaks our touch event handling).
+  documentBody.innerHTML = contentNode.innerHTML;
 
   if (target.type === 'bottom') {
     scrollToBottom();
@@ -519,8 +537,8 @@ export const handleInitialLoad = (platformOS: string, anchor: number, auth: Auth
     document.addEventListener('message', handleMessageEvent);
   }
 
+  processIncomingHtml(auth, documentBody);
   scrollToAnchor(anchor);
-  rewriteImageUrls(auth);
   sendScrollMessageIfListShort();
   scrollEventsDisabled = false;
 };

src/webview/js/rewriteImageUrls.js

@@ -28,11 +28,8 @@ const inlineApiRoutes: string[] = ['/user_uploads/', '/thumbnail?', '/avatar/'];
  *      than the document location.
  *   2. If the source URL names an endpoint known to require authentication,
  *      inject an API key into its query parameters.
- *
- * DEPRECATED: If no parent element is specified, transform every <img> in the
- * entire document.
  */
-const rewriteImageUrls = (auth: Auth, element: Element | Document = document) => {
+const rewriteImageUrls = (auth: Auth, element: Element) => {
   // The realm, parsed.
   const realm: URL = new URL(auth.realm);
 

src/webview/static/.gitignore

@@ -0,0 +1,3 @@
+# KaTeX directory will be [re]built by tools/built-webview
+/katex/*
+