From 1f24528350a958815ff12953932cc3cdbec93473 Mon Sep 17 00:00:00 2001
From: Cameron Sjo <cameronsjo@users.noreply.github.com>
Date: Thu, 14 May 2026 18:31:09 -0500
Subject: [PATCH 1/7] =?UTF-8?q?feat:=20improve=20markdown=20viewer=20?=
 =?UTF-8?q?=E2=80=94=20fix=20entities,=20add=20callouts,=20line=20numbers,?=
 =?UTF-8?q?=20selection?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

- Fix #4144: stop double-escaping HTML entities in inline code spans.
  marked v13 passes codespan content already escaped; remove redundant
  escapeHtml() call and add unescapeHtml() for path detection.

- Add GitHub-style callout/admonition blocks (> [!NOTE], > [!TIP],
  > [!IMPORTANT], > [!WARNING], > [!CAUTION]) with colored left borders
  and Octicon SVG icons.

- Add line number gutter to fenced code blocks. Numbers are non-selectable
  so copy grabs only code content.

- Fix #2591: Cmd+A / Ctrl+A now selects the entire document content
  instead of just the focused paragraph.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 Resources/markdown-viewer/shell.html | 105 ++++++++++++++++++++++++++-
 1 file changed, 102 insertions(+), 3 deletions(-)

diff --git a/Resources/markdown-viewer/shell.html b/Resources/markdown-viewer/shell.html
index c4f0bd7553bf..b5f1e6f36442 100644
--- a/Resources/markdown-viewer/shell.html
+++ b/Resources/markdown-viewer/shell.html
@@ -50,6 +50,33 @@
   padding: 0; /* hljs class already pads */
   overflow: auto;
 }
+/* Line number gutter for fenced code blocks. */
+pre.cmux-code-with-lines {
+  display: flex;
+  flex-direction: row;
+}
+pre.cmux-code-with-lines code.hljs {
+  flex: 1;
+  min-width: 0;
+}
+.cmux-line-gutter {
+  display: flex;
+  flex-direction: column;
+  padding: 16px 0;
+  padding-right: 12px;
+  padding-left: 12px;
+  border-right: 1px solid var(--borderColor-muted);
+  text-align: right;
+  -webkit-user-select: none;
+  user-select: none;
+  flex-shrink: 0;
+}
+.cmux-line-num {
+  font-size: 12px;
+  line-height: 1.5;
+  color: var(--fgColor-muted);
+  opacity: 0.55;
+}
 .cmux-code-block {
   position: relative;
   margin: 0 0 16px 0;
@@ -225,6 +252,39 @@
   margin: 0.6em 0;
   text-align: left;
 }
+/* GitHub-style callout/admonition blocks. */
+.cmux-callout {
+  padding: 12px 16px;
+  margin: 0 0 16px 0;
+  border-left: 4px solid;
+  border-radius: 6px;
+  background: var(--bgColor-muted);
+}
+.cmux-callout-title {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  font-weight: 600;
+  font-size: 14px;
+  margin-bottom: 6px;
+}
+.cmux-callout-title svg {
+  flex-shrink: 0;
+  width: 16px;
+  height: 16px;
+}
+.cmux-callout-body > :first-child { margin-top: 0; }
+.cmux-callout-body > :last-child { margin-bottom: 0; }
+.cmux-callout-note   { border-left-color: #539bf5; }
+.cmux-callout-tip    { border-left-color: #3fb950; }
+.cmux-callout-important { border-left-color: #a371f7; }
+.cmux-callout-warning { border-left-color: #d29922; }
+.cmux-callout-caution { border-left-color: #f85149; }
+.cmux-callout-note .cmux-callout-title   { color: #539bf5; }
+.cmux-callout-tip .cmux-callout-title    { color: #3fb950; }
+.cmux-callout-important .cmux-callout-title { color: #a371f7; }
+.cmux-callout-warning .cmux-callout-title { color: #d29922; }
+.cmux-callout-caution .cmux-callout-title { color: #f85149; }
 /* Custom selection color that reads on both themes. */
 ::selection { background: rgba(56, 139, 253, 0.4); }
 /* Smooth-anchor scroll for heading links. */
@@ -1012,8 +1072,9 @@
     pedantic: false,
     renderer: {
       codespan(code) {
-        var raw = code || '';
-        return '<code>' + escapeHtml(raw) + '</code>';
+        // marked v13 passes code already HTML-escaped; don't double-escape.
+        // Markdown-file links are detected later in markMarkdownFileLinks().
+        return '<code>' + (code || '') + '</code>';
       },
       code(code, infostring, escaped) {
         var raw = code || '';
@@ -1044,7 +1105,12 @@
           highlighted = escapeHtml(raw);
         }
         var langClass = langName ? ' language-' + langName : '';
-        return '<pre><code class="hljs' + langClass + '">' + highlighted + '</code></pre>\n';
+        var lineCount = (raw.match(/\n/g) || []).length + 1;
+        var gutter = '';
+        for (var ln = 1; ln <= lineCount; ln++) {
+          gutter += '<span class="cmux-line-num">' + ln + '</span>';
+        }
+        return '<pre class="cmux-code-with-lines"><span class="cmux-line-gutter" aria-hidden="true">' + gutter + '</span><code class="hljs' + langClass + '">' + highlighted + '</code></pre>\n';
       },
       heading(text, level, raw) {
         var slug = uniqueHeadingSlug(raw);
@@ -1056,6 +1122,26 @@
           html += ' title="' + escapeAttribute(decodeHTML(title)) + '"';
         }
         return html + '>' + sanitizeLinkLabelHTML(text) + '</a>';
+      },
+      blockquote(quote) {
+        var calloutMatch = (quote || '').match(/^<p>\[!(NOTE|TIP|IMPORTANT|WARNING|CAUTION)\]/i);
+        if (!calloutMatch) {
+          return '<blockquote>\n' + quote + '</blockquote>\n';
+        }
+        var type = calloutMatch[1].toLowerCase();
+        var body = quote.replace(calloutMatch[0], '<p>').replace(/^<p>\s*(?:<br\s*\/?>)?\s*/, '<p>');
+        var icons = {
+          note: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z"/></svg>',
+          tip: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M8 1.5c-2.363 0-4 1.69-4 3.75 0 .984.424 1.625.984 2.304l.214.253c.223.264.47.556.673.848.284.411.537.896.621 1.49a.75.75 0 0 1-1.484.211c-.04-.282-.163-.547-.37-.847a8.456 8.456 0 0 0-.542-.68c-.084-.1-.173-.205-.268-.32C3.201 7.75 2.5 6.766 2.5 5.25 2.5 2.31 4.863.5 8 .5s5.5 1.81 5.5 4.75c0 1.516-.701 2.5-1.328 3.259a10.8 10.8 0 0 0-.268.32c-.207.245-.383.453-.542.68-.207.3-.33.565-.37.847a.751.751 0 0 1-1.485-.212c.084-.593.337-1.078.621-1.489.203-.292.45-.584.673-.848.075-.088.147-.173.213-.253.561-.679.985-1.32.985-2.304 0-2.06-1.637-3.75-4-3.75ZM5.75 12h4.5a.75.75 0 0 1 0 1.5h-4.5a.75.75 0 0 1 0-1.5ZM6 15.25a.75.75 0 0 1 .75-.75h2.5a.75.75 0 0 1 0 1.5h-2.5a.75.75 0 0 1-.75-.75Z"/></svg>',
+          important: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M0 1.75C0 .784.784 0 1.75 0h12.5C15.216 0 16 .784 16 1.75v9.5A1.75 1.75 0 0 1 14.25 13H8.06l-2.573 2.573A1.458 1.458 0 0 1 3 14.543V13H1.75A1.75 1.75 0 0 1 0 11.25Zm1.75-.25a.25.25 0 0 0-.25.25v9.5c0 .138.112.25.25.25h2a.75.75 0 0 1 .75.75v2.19l2.72-2.72a.749.749 0 0 1 .53-.22h6.5a.25.25 0 0 0 .25-.25v-9.5a.25.25 0 0 0-.25-.25Zm7 2.25v2.5a.75.75 0 0 1-1.5 0v-2.5a.75.75 0 0 1 1.5 0ZM9 9a1 1 0 1 1-2 0 1 1 0 0 1 2 0Z"/></svg>',
+          warning: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M6.457 1.047c.659-1.234 2.427-1.234 3.086 0l6.082 11.378A1.75 1.75 0 0 1 14.082 15H1.918a1.75 1.75 0 0 1-1.543-2.575Zm1.763.707a.25.25 0 0 0-.44 0L1.698 13.132a.25.25 0 0 0 .22.368h12.164a.25.25 0 0 0 .22-.368Zm.53 3.996v2.5a.75.75 0 0 1-1.5 0v-2.5a.75.75 0 0 1 1.5 0ZM9 11a1 1 0 1 1-2 0 1 1 0 0 1 2 0Z"/></svg>',
+          caution: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M4.47.22A.749.749 0 0 1 5 0h6c.199 0 .389.079.53.22l4.25 4.25c.141.14.22.331.22.53v6a.749.749 0 0 1-.22.53l-4.25 4.25A.749.749 0 0 1 11 16H5a.749.749 0 0 1-.53-.22L.22 11.53A.749.749 0 0 1 0 11V5c0-.199.079-.389.22-.53Zm.84 1.28L1.5 5.31v5.38l3.81 3.81h5.38l3.81-3.81V5.31L10.69 1.5ZM8 4a.75.75 0 0 1 .75.75v3.5a.75.75 0 0 1-1.5 0v-3.5A.75.75 0 0 1 8 4Zm0 8a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z"/></svg>'
+        };
+        var title = type.charAt(0).toUpperCase() + type.slice(1);
+        return '<div class="cmux-callout cmux-callout-' + type + '">'
+          + '<div class="cmux-callout-title">' + (icons[type] || '') + ' ' + title + '</div>'
+          + '<div class="cmux-callout-body">' + body + '</div>'
+          + '</div>\n';
       }
     }
   });
@@ -1825,6 +1911,19 @@
     target: document,
     getScroller: markdownScroller
   });
+
+  // Fix Cmd+A / Ctrl+A to select the entire document content rather
+  // than just the focused paragraph (WebKit default behavior).
+  document.addEventListener('keydown', function(ev) {
+    if ((ev.metaKey || ev.ctrlKey) && !ev.shiftKey && !ev.altKey && ev.key === 'a') {
+      ev.preventDefault();
+      var range = document.createRange();
+      range.selectNodeContents(contentEl);
+      var sel = window.getSelection();
+      sel.removeAllRanges();
+      sel.addRange(range);
+    }
+  });
 })();
 </script>
 </body>

From 291d4be25268acae5103b826ce5bb6d5d1f0cb18 Mon Sep 17 00:00:00 2001
From: Cameron Sjo <cameronsjo@users.noreply.github.com>
Date: Thu, 14 May 2026 18:36:38 -0500
Subject: [PATCH 2/7] =?UTF-8?q?perf:=20optimize=20markdown=20render=20path?=
 =?UTF-8?q?=20=E2=80=94=20hash=20diff,=20sanitizer=20fast-path,=20debounce?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Three performance improvements to the markdown viewer render cycle:

- Replace O(n) innerHTML string comparison with FNV-1a hash check.
  On a 200KB document, this cuts the "nothing changed" path from
  milliseconds of string comparison to microseconds of integer compare.

- Add regex pre-check before the sanitizer TreeWalker. Most markdown
  files produce no dangerous HTML (no <script>, no onclick=, no
  javascript: URLs), so the full DOM walk is skipped entirely on the
  happy path. Sanitizer still fires when suspicious patterns are found.

- Debounce rapid re-renders with 80ms delay. First render is immediate;
  subsequent calls within the window are collapsed. Editors that do
  atomic write-rename (vim, VS Code) fire the file watcher 2-3 times
  in quick succession — this collapses them into one render.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 Resources/markdown-viewer/shell.html | 44 ++++++++++++++++++++++++++--
 1 file changed, 41 insertions(+), 3 deletions(-)

diff --git a/Resources/markdown-viewer/shell.html b/Resources/markdown-viewer/shell.html
index b5f1e6f36442..2183c5953067 100644
--- a/Resources/markdown-viewer/shell.html
+++ b/Resources/markdown-viewer/shell.html
@@ -340,6 +340,18 @@
     return value;
   }
 
+  // FNV-1a hash for fast content-change detection. Uses 32-bit arithmetic
+  // (safe in JS) to avoid the O(n) innerHTML string comparison on re-render.
+  function fnv1aHash(str) {
+    var hash = 0x811c9dc5;
+    for (var i = 0, len = str.length; i < len; i++) {
+      hash ^= str.charCodeAt(i);
+      hash = (hash * 0x01000193) >>> 0;
+    }
+    return hash;
+  }
+  var lastRenderedHash = 0;
+
   function escapeHtml(s) {
     // Let WebKit's HTML serializer do escaping rather than trying
     // to keep a hand-written entity table correct.
@@ -945,6 +957,14 @@
     }
   }
 
+  // Fast pre-check: most markdown files produce HTML with no dangerous
+  // elements or attributes. This regex tests for patterns the full
+  // TreeWalker sanitizer would act on — if none match, skip the walk.
+  var sanitizationPattern = /<(?:script|iframe|object|embed|link(?:\s)|meta|base|form|button|textarea|select|option|svg|math)\b|<input\b(?![^>]*type\s*=\s*["']?checkbox)|(?:\son[a-z]+=|\bstyle\s*=|\bsrcdoc\s*=|\bautofocus\b|\bformaction\s*=|\bxlink:href\s*=)|(?:href|src)\s*=\s*["']?\s*(?:javascript|vbscript|data):/i;
+  function needsSanitization(html) {
+    return sanitizationPattern.test(html || '');
+  }
+
   function sanitizeRenderedHTML(html) {
     var template = document.createElement('template');
     template.innerHTML = String(html || '');
@@ -1783,15 +1803,22 @@
     openMarkdownCandidate(rawPath);
   }, true);
 
-  window.__cmuxRenderMarkdown = function(md) {
+  // Debounce rapid re-renders (e.g. atomic write-rename fires watcher
+  // twice in quick succession). 80ms is fast enough to feel instant but
+  // collapses bursts from editors like vim, emacs, and VS Code.
+  var renderTimer = null;
+  function doRender(md) {
     try {
       headingSlugCounts = Object.create(null);
       var documentParts = extractFrontmatter(md || '');
+      var parsed = marked.parse(documentParts.body || '');
       var html = renderFrontmatter(documentParts.frontmatter)
-        + sanitizeRenderedHTML(marked.parse(documentParts.body || ''));
-      var didReplaceContent = contentEl.innerHTML !== html;
+        + (needsSanitization(parsed) ? sanitizeRenderedHTML(parsed) : parsed);
+      var hash = fnv1aHash(html);
+      var didReplaceContent = hash !== lastRenderedHash;
       var scrollState = didReplaceContent ? captureMarkdownScrollState() : null;
       if (didReplaceContent) {
+        lastRenderedHash = hash;
         contentEl.innerHTML = html;
       }
       rewriteLocalImageSources();
@@ -1806,6 +1833,17 @@
         + 'markdown render error: ' + escapeHtml(String((e && e.message) || e))
         + '</div><pre><code>' + escapeHtml(md || '') + '</code></pre>';
     }
+  }
+  window.__cmuxRenderMarkdown = function(md) {
+    if (renderTimer) { clearTimeout(renderTimer); }
+    if (lastRenderedHash === 0) {
+      doRender(md);
+      return;
+    }
+    renderTimer = setTimeout(function() {
+      renderTimer = null;
+      doRender(md);
+    }, 80);
   };
 
   function cleanRenderedContentClone(options) {

From d960f894d95c7fe1e1768a1068ff6534324ec91b Mon Sep 17 00:00:00 2001
From: Cameron Sjo <cameronsjo@users.noreply.github.com>
Date: Tue, 19 May 2026 10:00:24 -0500
Subject: [PATCH 3/7] fix(markdown-viewer): address review findings (round 1)

Addresses the six findings from CodeRabbit and Greptile on PR #4185:

1. Callout SVGs are no longer stripped by their own sanitizer. The icons
   are kept out of the rendered HTML pipeline and injected from
   postProcessSpecialBlocks() via a new injectCalloutIcons() helper. The
   blockquote renderer emits a data-cmux-callout-type placeholder whose
   value comes from a whitelisted capture group, so the attribute is
   safe even after a sanitization pass. (Greptile P1)

2. The fast-path sanitization regex now triggers on any character
   reference inside an href/src attribute value (e.g. "&#106;avascript:"),
   so entity-encoded protocol smuggling falls through to the full
   sanitizer (which already decodes via attribute parsing). (Greptile P1
   security)

3. Sanitization regex tightened: "<link\b" replaces "<link(?:\s)" so a
   bare "<link>" is caught, and "\son[a-z]+\s*=" allows whitespace before
   "=", so attributes like "onload =" are detected. (CodeRabbit critical)

4. lastRenderedHash now uses a null sentinel instead of 0. fnv1aHash can
   legitimately produce 0 (FNV-1a 32-bit, post ">>> 0"), which previously
   collided with the "not yet rendered" check. (Greptile P2)

5. Code-block line counting strips exactly one trailing newline before
   counting, removing the phantom dangling line number when raw code
   ends with "\n" (which is the common case from marked). (Greptile P2)

6. The viewer's Cmd/Ctrl+A handler matches on ev.code === "KeyA" instead
   of ev.key === "a" so the shortcut works regardless of keyboard layout
   or shift state. (CodeRabbit minor)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 Resources/markdown-viewer/shell.html | 69 ++++++++++++++++++++++------
 1 file changed, 56 insertions(+), 13 deletions(-)

diff --git a/Resources/markdown-viewer/shell.html b/Resources/markdown-viewer/shell.html
index 2183c5953067..82d0b503096e 100644
--- a/Resources/markdown-viewer/shell.html
+++ b/Resources/markdown-viewer/shell.html
@@ -350,7 +350,9 @@
     }
     return hash;
   }
-  var lastRenderedHash = 0;
+  // Sentinel must not collide with any valid fnv1aHash() output. fnv1aHash
+  // ends with `>>> 0`, which produces 0..2^32-1; null sits outside that range.
+  var lastRenderedHash = null;
 
   function escapeHtml(s) {
     // Let WebKit's HTML serializer do escaping rather than trying
@@ -960,7 +962,18 @@
   // Fast pre-check: most markdown files produce HTML with no dangerous
   // elements or attributes. This regex tests for patterns the full
   // TreeWalker sanitizer would act on — if none match, skip the walk.
-  var sanitizationPattern = /<(?:script|iframe|object|embed|link(?:\s)|meta|base|form|button|textarea|select|option|svg|math)\b|<input\b(?![^>]*type\s*=\s*["']?checkbox)|(?:\son[a-z]+=|\bstyle\s*=|\bsrcdoc\s*=|\bautofocus\b|\bformaction\s*=|\bxlink:href\s*=)|(?:href|src)\s*=\s*["']?\s*(?:javascript|vbscript|data):/i;
+  //
+  // Notes on each alternation:
+  //   <link\b ...                  word-boundary, not <link(?:\s) — catches
+  //                                <link> with no following whitespace.
+  //   \son[a-z]+\s*=               whitespace before `=` is tolerated by
+  //                                browsers, so `onload =` must still match.
+  //   (?:href|src) ... &           any character reference inside an href/src
+  //                                value (e.g. `&#106;avascript:`) forces the
+  //                                full sanitizer; entity decoding happens at
+  //                                attribute-parse time, so the slow path
+  //                                gets the resolved value via attr.value.
+  var sanitizationPattern = /<(?:script|iframe|object|embed|link\b|meta|base|form|button|textarea|select|option|svg|math)\b|<input\b(?![^>]*type\s*=\s*["']?checkbox)|(?:\son[a-z]+\s*=|\bstyle\s*=|\bsrcdoc\s*=|\bautofocus\b|\bformaction\s*=|\bxlink:href\s*=)|(?:href|src)\s*=\s*["']?\s*(?:javascript|vbscript|data):|(?:href|src)\s*=\s*["'][^"']*&/i;
   function needsSanitization(html) {
     return sanitizationPattern.test(html || '');
   }
@@ -1125,7 +1138,11 @@
           highlighted = escapeHtml(raw);
         }
         var langClass = langName ? ' language-' + langName : '';
-        var lineCount = (raw.match(/\n/g) || []).length + 1;
+        // Strip exactly one trailing newline before counting lines so code
+        // blocks that end in "\n" (the common case from marked) don't render
+        // a phantom empty line number in the gutter.
+        var rawForLineCount = raw.replace(/\r?\n$/, '');
+        var lineCount = (rawForLineCount.match(/\n/g) || []).length + 1;
         var gutter = '';
         for (var ln = 1; ln <= lineCount; ln++) {
           gutter += '<span class="cmux-line-num">' + ln + '</span>';
@@ -1150,16 +1167,14 @@
         }
         var type = calloutMatch[1].toLowerCase();
         var body = quote.replace(calloutMatch[0], '<p>').replace(/^<p>\s*(?:<br\s*\/?>)?\s*/, '<p>');
-        var icons = {
-          note: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z"/></svg>',
-          tip: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M8 1.5c-2.363 0-4 1.69-4 3.75 0 .984.424 1.625.984 2.304l.214.253c.223.264.47.556.673.848.284.411.537.896.621 1.49a.75.75 0 0 1-1.484.211c-.04-.282-.163-.547-.37-.847a8.456 8.456 0 0 0-.542-.68c-.084-.1-.173-.205-.268-.32C3.201 7.75 2.5 6.766 2.5 5.25 2.5 2.31 4.863.5 8 .5s5.5 1.81 5.5 4.75c0 1.516-.701 2.5-1.328 3.259a10.8 10.8 0 0 0-.268.32c-.207.245-.383.453-.542.68-.207.3-.33.565-.37.847a.751.751 0 0 1-1.485-.212c.084-.593.337-1.078.621-1.489.203-.292.45-.584.673-.848.075-.088.147-.173.213-.253.561-.679.985-1.32.985-2.304 0-2.06-1.637-3.75-4-3.75ZM5.75 12h4.5a.75.75 0 0 1 0 1.5h-4.5a.75.75 0 0 1 0-1.5ZM6 15.25a.75.75 0 0 1 .75-.75h2.5a.75.75 0 0 1 0 1.5h-2.5a.75.75 0 0 1-.75-.75Z"/></svg>',
-          important: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M0 1.75C0 .784.784 0 1.75 0h12.5C15.216 0 16 .784 16 1.75v9.5A1.75 1.75 0 0 1 14.25 13H8.06l-2.573 2.573A1.458 1.458 0 0 1 3 14.543V13H1.75A1.75 1.75 0 0 1 0 11.25Zm1.75-.25a.25.25 0 0 0-.25.25v9.5c0 .138.112.25.25.25h2a.75.75 0 0 1 .75.75v2.19l2.72-2.72a.749.749 0 0 1 .53-.22h6.5a.25.25 0 0 0 .25-.25v-9.5a.25.25 0 0 0-.25-.25Zm7 2.25v2.5a.75.75 0 0 1-1.5 0v-2.5a.75.75 0 0 1 1.5 0ZM9 9a1 1 0 1 1-2 0 1 1 0 0 1 2 0Z"/></svg>',
-          warning: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M6.457 1.047c.659-1.234 2.427-1.234 3.086 0l6.082 11.378A1.75 1.75 0 0 1 14.082 15H1.918a1.75 1.75 0 0 1-1.543-2.575Zm1.763.707a.25.25 0 0 0-.44 0L1.698 13.132a.25.25 0 0 0 .22.368h12.164a.25.25 0 0 0 .22-.368Zm.53 3.996v2.5a.75.75 0 0 1-1.5 0v-2.5a.75.75 0 0 1 1.5 0ZM9 11a1 1 0 1 1-2 0 1 1 0 0 1 2 0Z"/></svg>',
-          caution: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M4.47.22A.749.749 0 0 1 5 0h6c.199 0 .389.079.53.22l4.25 4.25c.141.14.22.331.22.53v6a.749.749 0 0 1-.22.53l-4.25 4.25A.749.749 0 0 1 11 16H5a.749.749 0 0 1-.53-.22L.22 11.53A.749.749 0 0 1 0 11V5c0-.199.079-.389.22-.53Zm.84 1.28L1.5 5.31v5.38l3.81 3.81h5.38l3.81-3.81V5.31L10.69 1.5ZM8 4a.75.75 0 0 1 .75.75v3.5a.75.75 0 0 1-1.5 0v-3.5A.75.75 0 0 1 8 4Zm0 8a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z"/></svg>'
-        };
         var title = type.charAt(0).toUpperCase() + type.slice(1);
+        // Emit a data-attribute placeholder; the icon <svg> is injected
+        // after sanitization by injectCalloutIcons() so the sanitizer
+        // (which strips all <svg> elements as a hardening measure) cannot
+        // remove our own trusted icons. `type` comes from the allow-listed
+        // capture group above, so the attribute value is safe.
         return '<div class="cmux-callout cmux-callout-' + type + '">'
-          + '<div class="cmux-callout-title">' + (icons[type] || '') + ' ' + title + '</div>'
+          + '<div class="cmux-callout-title" data-cmux-callout-type="' + type + '">' + title + '</div>'
           + '<div class="cmux-callout-body">' + body + '</div>'
           + '</div>\n';
       }
@@ -1639,7 +1654,31 @@
     });
   }
 
+  // Octicon SVGs for GitHub-style callouts. Kept out of the rendered HTML
+  // pipeline and injected post-sanitization so the sanitizer's blanket
+  // <svg> strip does not remove our own trusted icons. Markup is constant
+  // and never blended with user-supplied content.
+  var calloutIcons = {
+    note: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M0 8a8 8 0 1 1 16 0A8 8 0 0 1 0 8Zm8-6.5a6.5 6.5 0 1 0 0 13 6.5 6.5 0 0 0 0-13ZM6.5 7.75A.75.75 0 0 1 7.25 7h1a.75.75 0 0 1 .75.75v2.75h.25a.75.75 0 0 1 0 1.5h-2a.75.75 0 0 1 0-1.5h.25v-2h-.25a.75.75 0 0 1-.75-.75ZM8 6a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z"/></svg>',
+    tip: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M8 1.5c-2.363 0-4 1.69-4 3.75 0 .984.424 1.625.984 2.304l.214.253c.223.264.47.556.673.848.284.411.537.896.621 1.49a.75.75 0 0 1-1.484.211c-.04-.282-.163-.547-.37-.847a8.456 8.456 0 0 0-.542-.68c-.084-.1-.173-.205-.268-.32C3.201 7.75 2.5 6.766 2.5 5.25 2.5 2.31 4.863.5 8 .5s5.5 1.81 5.5 4.75c0 1.516-.701 2.5-1.328 3.259a10.8 10.8 0 0 0-.268.32c-.207.245-.383.453-.542.68-.207.3-.33.565-.37.847a.751.751 0 0 1-1.485-.212c.084-.593.337-1.078.621-1.489.203-.292.45-.584.673-.848.075-.088.147-.173.213-.253.561-.679.985-1.32.985-2.304 0-2.06-1.637-3.75-4-3.75ZM5.75 12h4.5a.75.75 0 0 1 0 1.5h-4.5a.75.75 0 0 1 0-1.5ZM6 15.25a.75.75 0 0 1 .75-.75h2.5a.75.75 0 0 1 0 1.5h-2.5a.75.75 0 0 1-.75-.75Z"/></svg>',
+    important: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M0 1.75C0 .784.784 0 1.75 0h12.5C15.216 0 16 .784 16 1.75v9.5A1.75 1.75 0 0 1 14.25 13H8.06l-2.573 2.573A1.458 1.458 0 0 1 3 14.543V13H1.75A1.75 1.75 0 0 1 0 11.25Zm1.75-.25a.25.25 0 0 0-.25.25v9.5c0 .138.112.25.25.25h2a.75.75 0 0 1 .75.75v2.19l2.72-2.72a.749.749 0 0 1 .53-.22h6.5a.25.25 0 0 0 .25-.25v-9.5a.25.25 0 0 0-.25-.25Zm7 2.25v2.5a.75.75 0 0 1-1.5 0v-2.5a.75.75 0 0 1 1.5 0ZM9 9a1 1 0 1 1-2 0 1 1 0 0 1 2 0Z"/></svg>',
+    warning: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M6.457 1.047c.659-1.234 2.427-1.234 3.086 0l6.082 11.378A1.75 1.75 0 0 1 14.082 15H1.918a1.75 1.75 0 0 1-1.543-2.575Zm1.763.707a.25.25 0 0 0-.44 0L1.698 13.132a.25.25 0 0 0 .22.368h12.164a.25.25 0 0 0 .22-.368Zm.53 3.996v2.5a.75.75 0 0 1-1.5 0v-2.5a.75.75 0 0 1 1.5 0ZM9 11a1 1 0 1 1-2 0 1 1 0 0 1 2 0Z"/></svg>',
+    caution: '<svg viewBox="0 0 16 16" fill="currentColor"><path d="M4.47.22A.749.749 0 0 1 5 0h6c.199 0 .389.079.53.22l4.25 4.25c.141.14.22.331.22.53v6a.749.749 0 0 1-.22.53l-4.25 4.25A.749.749 0 0 1 11 16H5a.749.749 0 0 1-.53-.22L.22 11.53A.749.749 0 0 1 0 11V5c0-.199.079-.389.22-.53Zm.84 1.28L1.5 5.31v5.38l3.81 3.81h5.38l3.81-3.81V5.31L10.69 1.5ZM8 4a.75.75 0 0 1 .75.75v3.5a.75.75 0 0 1-1.5 0v-3.5A.75.75 0 0 1 8 4Zm0 8a1 1 0 1 1 0-2 1 1 0 0 1 0 2Z"/></svg>'
+  };
+
+  function injectCalloutIcons() {
+    contentEl.querySelectorAll('.cmux-callout-title[data-cmux-callout-type]').forEach(function(titleEl) {
+      if (titleEl.querySelector('svg')) { return; }
+      var type = String(titleEl.getAttribute('data-cmux-callout-type') || '').toLowerCase();
+      var icon = calloutIcons[type];
+      if (icon) {
+        titleEl.insertAdjacentHTML('afterbegin', icon + ' ');
+      }
+    });
+  }
+
   function postProcessSpecialBlocks() {
+    injectCalloutIcons();
     if (contentEl.querySelector('.cmux-mermaid:not([data-rendered])')) {
       loadLib('mermaid', renderMermaidBlocks);
     }
@@ -1836,7 +1875,7 @@
   }
   window.__cmuxRenderMarkdown = function(md) {
     if (renderTimer) { clearTimeout(renderTimer); }
-    if (lastRenderedHash === 0) {
+    if (lastRenderedHash === null) {
       doRender(md);
       return;
     }
@@ -1952,8 +1991,12 @@
 
   // Fix Cmd+A / Ctrl+A to select the entire document content rather
   // than just the focused paragraph (WebKit default behavior).
+  //
+  // Match on ev.code === 'KeyA' (physical key position) rather than
+  // ev.key === 'a' so the shortcut works regardless of keyboard layout
+  // and modifier-induced case changes.
   document.addEventListener('keydown', function(ev) {
-    if ((ev.metaKey || ev.ctrlKey) && !ev.shiftKey && !ev.altKey && ev.key === 'a') {
+    if ((ev.metaKey || ev.ctrlKey) && !ev.shiftKey && !ev.altKey && ev.code === 'KeyA') {
       ev.preventDefault();
       var range = document.createRange();
       range.selectNodeContents(contentEl);

From 39777bcddb1765552f140d7cc5c18b17459f14c4 Mon Sep 17 00:00:00 2001
From: Cameron Sjo <cameronsjo@users.noreply.github.com>
Date: Tue, 19 May 2026 19:42:31 -0500
Subject: [PATCH 4/7] fix(markdown-viewer): keep remote-image privacy gate on
 sanitizer fast-path
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

The render perf work added a needsSanitization() fast-path that skips
sanitizeRenderedHTML() when no dangerous pattern matches. After rebasing
onto main (which landed #4288's remote-image privacy feature), this
created a gap: #4288 strips a remote <img> src into data-cmux-remote-src
*inside* sanitizeRenderedHTML — which runs on an inert <template>, so the
image never fetches. A document containing only a plain remote image
matched none of the fast-path patterns, so sanitization was skipped, the
live src survived into the live DOM, and the browser auto-fetched the
remote image — defeating the "don't auto-load remote images" gate.

Add an <img ... src=...http(s): alternative to the fast-path regex so any
remote image forces the full sanitizer. Local/relative and file images
stay on the fast path (rewriteLocalImageSources scans img[src] directly,
independent of sanitization); data: images already trigger via the
existing data: alternative.

Verified against a 22-case regex suite covering the original review
vectors plus remote/local/data image and lazy data-src cases.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 Resources/markdown-viewer/shell.html | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/Resources/markdown-viewer/shell.html b/Resources/markdown-viewer/shell.html
index 82d0b503096e..8e566c55025b 100644
--- a/Resources/markdown-viewer/shell.html
+++ b/Resources/markdown-viewer/shell.html
@@ -973,7 +973,15 @@
   //                                full sanitizer; entity decoding happens at
   //                                attribute-parse time, so the slow path
   //                                gets the resolved value via attr.value.
-  var sanitizationPattern = /<(?:script|iframe|object|embed|link\b|meta|base|form|button|textarea|select|option|svg|math)\b|<input\b(?![^>]*type\s*=\s*["']?checkbox)|(?:\son[a-z]+\s*=|\bstyle\s*=|\bsrcdoc\s*=|\bautofocus\b|\bformaction\s*=|\bxlink:href\s*=)|(?:href|src)\s*=\s*["']?\s*(?:javascript|vbscript|data):|(?:href|src)\s*=\s*["'][^"']*&/i;
+  //   <img ... src=...http(s):     remote images must reach sanitizeRenderedHTML
+  //                                (which runs on an inert <template>) so their
+  //                                live src is stripped into data-cmux-remote-src
+  //                                before insertion — otherwise the browser
+  //                                auto-fetches them and the "don't auto-load
+  //                                remote images" privacy gate is bypassed.
+  //                                Local (relative/file) images stay on the fast
+  //                                path; rewriteLocalImageSources handles them.
+  var sanitizationPattern = /<(?:script|iframe|object|embed|link\b|meta|base|form|button|textarea|select|option|svg|math)\b|<input\b(?![^>]*type\s*=\s*["']?checkbox)|<img\b[^>]*\ssrc\s*=\s*["']?\s*https?:|(?:\son[a-z]+\s*=|\bstyle\s*=|\bsrcdoc\s*=|\bautofocus\b|\bformaction\s*=|\bxlink:href\s*=)|(?:href|src)\s*=\s*["']?\s*(?:javascript|vbscript|data):|(?:href|src)\s*=\s*["'][^"']*&/i;
   function needsSanitization(html) {
     return sanitizationPattern.test(html || '');
   }

From 2f1095011a1769bfb2d3e1d4aab32336bed3ffee Mon Sep 17 00:00:00 2001
From: Cameron Sjo <cameronsjo@users.noreply.github.com>
Date: Tue, 19 May 2026 19:54:25 -0500
Subject: [PATCH 5/7] fix(markdown-viewer): address review findings (round 2)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Three findings from CodeRabbit's re-review of the rebased branch:

1. FNV-1a hash now uses Math.imul(hash, 0x01000193) instead of
   `hash * 0x01000193`. Plain `*` is an IEEE-754 double multiply; once the
   product exceeds 2^53 the `>>> 0` truncation can't recover the correct
   32-bit value, so the content-change hash could miss real edits. (Major)

2. The needsSanitization() fast-path is now a strict superset of what
   sanitizeRenderedHTML() strips. Added the tags that #4288 introduced into
   blockedTags but were missing from the regex (style, audio, video,
   picture, source, track), and broadened the entity-reference branch to
   match unquoted href/src values too (e.g. `href=&#106;avascript:`), so
   nothing the slow path would strip can skip it. (Critical)

3. Line-number gutters (.cmux-line-gutter) are stripped in
   cleanRenderedContentClone(), so exported HTML and copied plain text from
   __cmuxRenderedHTML()/__cmuxRenderedText() no longer carry "1 2 3 …". (Major)

Verified against a 26-case regex suite covering the new tag/entity cases,
the original review vectors, and safe-content negatives.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 Resources/markdown-viewer/shell.html | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/Resources/markdown-viewer/shell.html b/Resources/markdown-viewer/shell.html
index 8e566c55025b..a9243063baa8 100644
--- a/Resources/markdown-viewer/shell.html
+++ b/Resources/markdown-viewer/shell.html
@@ -346,7 +346,9 @@
     var hash = 0x811c9dc5;
     for (var i = 0, len = str.length; i < len; i++) {
       hash ^= str.charCodeAt(i);
-      hash = (hash * 0x01000193) >>> 0;
+      // Math.imul gives C-like 32-bit integer multiply; plain `*` would lose
+      // precision once the product exceeds 2^53, corrupting the hash.
+      hash = Math.imul(hash, 0x01000193) >>> 0;
     }
     return hash;
   }
@@ -981,7 +983,13 @@
   //                                remote images" privacy gate is bypassed.
   //                                Local (relative/file) images stay on the fast
   //                                path; rewriteLocalImageSources handles them.
-  var sanitizationPattern = /<(?:script|iframe|object|embed|link\b|meta|base|form|button|textarea|select|option|svg|math)\b|<input\b(?![^>]*type\s*=\s*["']?checkbox)|<img\b[^>]*\ssrc\s*=\s*["']?\s*https?:|(?:\son[a-z]+\s*=|\bstyle\s*=|\bsrcdoc\s*=|\bautofocus\b|\bformaction\s*=|\bxlink:href\s*=)|(?:href|src)\s*=\s*["']?\s*(?:javascript|vbscript|data):|(?:href|src)\s*=\s*["'][^"']*&/i;
+  //
+  // INVARIANT: this pattern must be a superset of everything
+  // sanitizeRenderedHTML() would act on. The tag list mirrors blockedTags
+  // (incl. style + media tags audio/video/picture/source/track), and the
+  // entity-reference branch matches quoted AND unquoted href/src values so
+  // forms like `href=&#106;avascript:` cannot skip the slow path.
+  var sanitizationPattern = /<(?:script|iframe|object|embed|link\b|meta|base|style|audio|video|picture|source|track|form|button|textarea|select|option|svg|math)\b|<input\b(?![^>]*type\s*=\s*["']?checkbox)|<img\b[^>]*\ssrc\s*=\s*["']?\s*https?:|(?:\son[a-z]+\s*=|\bstyle\s*=|\bsrcdoc\s*=|\bautofocus\b|\bformaction\s*=|\bxlink:href\s*=)|(?:href|src)\s*=\s*["']?\s*(?:javascript|vbscript|data):|(?:href|src)\s*=\s*(?:"[^"]*&|'[^']*&|[^"'\s>]*&)/i;
   function needsSanitization(html) {
     return sanitizationPattern.test(html || '');
   }
@@ -1901,6 +1909,11 @@
     clone.querySelectorAll('.cmux-code-copy-button').forEach(function(el) {
       el.remove();
     });
+    // Line-number gutters are viewer chrome, not content — drop them so
+    // exported HTML and copied plain text don't carry "1 2 3 …" line numbers.
+    clone.querySelectorAll('.cmux-line-gutter').forEach(function(el) {
+      el.remove();
+    });
     clone.querySelectorAll('.cmux-remote-image-placeholder').forEach(function(el) {
       el.remove();
     });

From b0c01dfc5441a9b5fe820ea2018491d2ec45e15c Mon Sep 17 00:00:00 2001
From: Cameron Sjo <cameronsjo@users.noreply.github.com>
Date: Tue, 19 May 2026 20:03:28 -0500
Subject: [PATCH 6/7] fix(markdown-viewer): align fast-path with sanitizer
 srcset/background (round 3)

cubic flagged that needsSanitization() omitted the `srcset` and
`background` attributes that sanitizeRenderedHTML() strips (lines 1001-1002),
violating the documented superset invariant. A document containing only
those attributes (e.g. <img srcset="https://...">) would skip the slow path,
leaving a remote-image privacy bypass. Added \bsrcset\s*= and
\bbackground\s*= to the attribute alternation; the fast-path attribute set
now matches the sanitizer's blocked-attribute list exactly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 Resources/markdown-viewer/shell.html | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Resources/markdown-viewer/shell.html b/Resources/markdown-viewer/shell.html
index a9243063baa8..84ca83a2f4fe 100644
--- a/Resources/markdown-viewer/shell.html
+++ b/Resources/markdown-viewer/shell.html
@@ -989,7 +989,7 @@
   // (incl. style + media tags audio/video/picture/source/track), and the
   // entity-reference branch matches quoted AND unquoted href/src values so
   // forms like `href=&#106;avascript:` cannot skip the slow path.
-  var sanitizationPattern = /<(?:script|iframe|object|embed|link\b|meta|base|style|audio|video|picture|source|track|form|button|textarea|select|option|svg|math)\b|<input\b(?![^>]*type\s*=\s*["']?checkbox)|<img\b[^>]*\ssrc\s*=\s*["']?\s*https?:|(?:\son[a-z]+\s*=|\bstyle\s*=|\bsrcdoc\s*=|\bautofocus\b|\bformaction\s*=|\bxlink:href\s*=)|(?:href|src)\s*=\s*["']?\s*(?:javascript|vbscript|data):|(?:href|src)\s*=\s*(?:"[^"]*&|'[^']*&|[^"'\s>]*&)/i;
+  var sanitizationPattern = /<(?:script|iframe|object|embed|link\b|meta|base|style|audio|video|picture|source|track|form|button|textarea|select|option|svg|math)\b|<input\b(?![^>]*type\s*=\s*["']?checkbox)|<img\b[^>]*\ssrc\s*=\s*["']?\s*https?:|(?:\son[a-z]+\s*=|\bstyle\s*=|\bsrcset\s*=|\bbackground\s*=|\bsrcdoc\s*=|\bautofocus\b|\bformaction\s*=|\bxlink:href\s*=)|(?:href|src)\s*=\s*["']?\s*(?:javascript|vbscript|data):|(?:href|src)\s*=\s*(?:"[^"]*&|'[^']*&|[^"'\s>]*&)/i;
   function needsSanitization(html) {
     return sanitizationPattern.test(html || '');
   }

From 820935414cf0168158ee2a1a7dcd72b80f2aa72f Mon Sep 17 00:00:00 2001
From: Cameron Sjo <cameronsjo@users.noreply.github.com>
Date: Tue, 19 May 2026 20:09:22 -0500
Subject: [PATCH 7/7] refactor(markdown-viewer): derive sanitizer fast-path
 from shared config
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Three consecutive review rounds flagged the needsSanitization() fast-path
drifting from sanitizeRenderedHTML() — first missing style/media tags, then
unquoted entity refs, then srcset/background. Root cause: the fast-path
regex and the sanitizer's blocked-tag/attr lists were maintained by hand in
two places, so every sanitizer change needed a mirror edit that was easy to
miss.

Make them share one source of truth:

- SANITIZER_BLOCKED_TAGS and SANITIZER_BLOCKED_ATTRS are now module-level
  constants. sanitizeRenderedHTML() consumes them directly (blockedTags
  alias + SANITIZER_BLOCKED_ATTRS.indexOf for attribute stripping).
- buildSanitizationPattern() generates the fast-path regex from the same
  lists, with SANITIZER_BOOLEAN_ATTRS marking valueless attributes
  (autofocus) so they match on a word boundary rather than `=`.
- Adding a tag or attribute now updates both the sanitizer and the
  fast-path together; they cannot drift.

The generated regex is behaviorally identical to the prior hand-written one.
Verified by running the in-file builder against a 30-case suite covering
every blocked tag, blocked attribute (incl. bare boolean autofocus), the
quoted/unquoted entity and active-protocol URL branches, remote vs local
images, and safe-content negatives.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
---
 Resources/markdown-viewer/shell.html | 122 +++++++++++++--------------
 1 file changed, 61 insertions(+), 61 deletions(-)

diff --git a/Resources/markdown-viewer/shell.html b/Resources/markdown-viewer/shell.html
index 84ca83a2f4fe..cb4cccc1014c 100644
--- a/Resources/markdown-viewer/shell.html
+++ b/Resources/markdown-viewer/shell.html
@@ -961,35 +961,63 @@
     }
   }
 
-  // Fast pre-check: most markdown files produce HTML with no dangerous
-  // elements or attributes. This regex tests for patterns the full
-  // TreeWalker sanitizer would act on — if none match, skip the walk.
-  //
-  // Notes on each alternation:
-  //   <link\b ...                  word-boundary, not <link(?:\s) — catches
-  //                                <link> with no following whitespace.
-  //   \son[a-z]+\s*=               whitespace before `=` is tolerated by
-  //                                browsers, so `onload =` must still match.
-  //   (?:href|src) ... &           any character reference inside an href/src
-  //                                value (e.g. `&#106;avascript:`) forces the
-  //                                full sanitizer; entity decoding happens at
-  //                                attribute-parse time, so the slow path
-  //                                gets the resolved value via attr.value.
-  //   <img ... src=...http(s):     remote images must reach sanitizeRenderedHTML
-  //                                (which runs on an inert <template>) so their
-  //                                live src is stripped into data-cmux-remote-src
-  //                                before insertion — otherwise the browser
-  //                                auto-fetches them and the "don't auto-load
-  //                                remote images" privacy gate is bypassed.
-  //                                Local (relative/file) images stay on the fast
-  //                                path; rewriteLocalImageSources handles them.
-  //
-  // INVARIANT: this pattern must be a superset of everything
-  // sanitizeRenderedHTML() would act on. The tag list mirrors blockedTags
-  // (incl. style + media tags audio/video/picture/source/track), and the
-  // entity-reference branch matches quoted AND unquoted href/src values so
-  // forms like `href=&#106;avascript:` cannot skip the slow path.
-  var sanitizationPattern = /<(?:script|iframe|object|embed|link\b|meta|base|style|audio|video|picture|source|track|form|button|textarea|select|option|svg|math)\b|<input\b(?![^>]*type\s*=\s*["']?checkbox)|<img\b[^>]*\ssrc\s*=\s*["']?\s*https?:|(?:\son[a-z]+\s*=|\bstyle\s*=|\bsrcset\s*=|\bbackground\s*=|\bsrcdoc\s*=|\bautofocus\b|\bformaction\s*=|\bxlink:href\s*=)|(?:href|src)\s*=\s*["']?\s*(?:javascript|vbscript|data):|(?:href|src)\s*=\s*(?:"[^"]*&|'[^']*&|[^"'\s>]*&)/i;
+  // Single source of truth for sanitization. Both the full TreeWalker
+  // sanitizer (sanitizeRenderedHTML) and the fast pre-check (needsSanitization)
+  // are derived from these lists, so the fast-path can never drift out of sync
+  // with what the sanitizer actually strips. Add a tag or attribute here once
+  // and both paths update together.
+  var SANITIZER_BLOCKED_TAGS = {
+    script: true, iframe: true, object: true, embed: true, link: true,
+    meta: true, base: true, style: true, audio: true, video: true,
+    picture: true, source: true, track: true, form: true, button: true,
+    textarea: true, select: true, option: true, svg: true, math: true
+  };
+  // Attributes the sanitizer strips outright. Any on* event handler is also
+  // stripped (handled by prefix, not listed here). `input` (checkbox-only) and
+  // remote <img> src are special-cased in both the sanitizer and the pattern.
+  var SANITIZER_BLOCKED_ATTRS = [
+    'style', 'background', 'srcset', 'srcdoc', 'autofocus', 'formaction', 'xlink:href'
+  ];
+  // Valueless-capable (boolean) attributes match on a word boundary in the
+  // fast-path regex rather than requiring `=` (e.g. bare `<button autofocus>`).
+  var SANITIZER_BOOLEAN_ATTRS = { autofocus: true };
+
+  function reEscapeForPattern(s) {
+    return String(s).replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
+  }
+
+  // Build the fast pre-check regex from the same lists the sanitizer uses.
+  // Most markdown produces HTML with nothing dangerous; when none of these
+  // alternations match we skip the TreeWalker walk entirely.
+  //   - tag list           mirrors SANITIZER_BLOCKED_TAGS
+  //   - <input ...checkbox  input is allowed only as a checkbox
+  //   - <img ...src=http(s) remote images must reach sanitizeRenderedHTML
+  //                         (which runs on an inert <template>) so the live src
+  //                         is stripped before the browser auto-fetches it —
+  //                         the "don't auto-load remote images" privacy gate.
+  //                         Local/relative images stay on the fast path.
+  //   - on* / blocked attrs mirror SANITIZER_BLOCKED_ATTRS
+  //   - javascript:/...     active-protocol href/src
+  //   - href/src ... &      any character reference (quoted or unquoted) in an
+  //                         href/src value, so entity-encoded payloads like
+  //                         `href=&#106;avascript:` can't skip the slow path.
+  function buildSanitizationPattern() {
+    var tags = Object.keys(SANITIZER_BLOCKED_TAGS).map(reEscapeForPattern).join('|');
+    var attrs = SANITIZER_BLOCKED_ATTRS.map(function(a) {
+      var esc = reEscapeForPattern(a);
+      return SANITIZER_BOOLEAN_ATTRS[a] ? '\\b' + esc + '\\b' : '\\b' + esc + '\\s*=';
+    }).join('|');
+    return new RegExp(
+      '<(?:' + tags + ')\\b' +
+      '|<input\\b(?![^>]*type\\s*=\\s*["\']?checkbox)' +
+      '|<img\\b[^>]*\\ssrc\\s*=\\s*["\']?\\s*https?:' +
+      '|(?:\\son[a-z]+\\s*=|' + attrs + ')' +
+      '|(?:href|src)\\s*=\\s*["\']?\\s*(?:javascript|vbscript|data):' +
+      '|(?:href|src)\\s*=\\s*(?:"[^"]*&|\'[^\']*&|[^"\'\\s>]*&)',
+      'i'
+    );
+  }
+  var sanitizationPattern = buildSanitizationPattern();
   function needsSanitization(html) {
     return sanitizationPattern.test(html || '');
   }
@@ -998,28 +1026,9 @@
     var template = document.createElement('template');
     template.innerHTML = String(html || '');
 
-    var blockedTags = {
-      script: true,
-      iframe: true,
-      object: true,
-      embed: true,
-      link: true,
-      style: true,
-      meta: true,
-      base: true,
-      audio: true,
-      form: true,
-      button: true,
-      picture: true,
-      source: true,
-      textarea: true,
-      track: true,
-      video: true,
-      select: true,
-      option: true,
-      svg: true,
-      math: true
-    };
+    // Reuse the shared block list so the sanitizer and the fast-path regex
+    // (buildSanitizationPattern) can never disagree about which tags to strip.
+    var blockedTags = SANITIZER_BLOCKED_TAGS;
 
     var walker = document.createTreeWalker(template.content, NodeFilter.SHOW_ELEMENT);
     var elements = [];
@@ -1056,16 +1065,7 @@
           el.removeAttribute(attr.name);
           return;
         }
-        if (
-          name.indexOf('on') === 0 ||
-          name === 'style' ||
-          name === 'background' ||
-          name === 'srcset' ||
-          name === 'srcdoc' ||
-          name === 'autofocus' ||
-          name === 'formaction' ||
-          name === 'xlink:href'
-        ) {
+        if (name.indexOf('on') === 0 || SANITIZER_BLOCKED_ATTRS.indexOf(name) !== -1) {
           el.removeAttribute(attr.name);
           return;
         }