The markdown fence regex /```([\s\S]*?)```/g had no line anchoring. A literal
triple backtick inside code block content (e.g. a regex with ``` in a lookbehind,
or a script that documents fences) terminated the outer fence at the wrong place.
The leaked tail then went through bold/italic/inline-code passes, eating `*`
characters as italic markers and emitting literal </strong> tags into the
rendered output.
CommonMark §4.5 requires that an opening code fence be the first non-whitespace
content of a line (up to 3 spaces of indent allowed) and that the closing fence
also start a line. This patch updates 3 sites + the Python mirror to use that
invariant:
static/ui.js:1559 renderMd() fenced-block stash (assistant messages)
static/ui.js:66 _renderUserFencedBlocks() (user messages)
static/ui.js:2599 _stripForTTS() (TTS speech pre-strip)
tests/test_sprint16.py Python mirror
Pattern: (^|\n)[ ]{0,3}```(?:([\s\S]*?)\n)?[ ]{0,3}```(?=\n|$)
The non-capturing (?:...\n)? group keeps empty fences (```\n```) working;
without it, a body+\n is required and the closing fence on the very next line
no longer matches. The lead group (^|\n) is prefixed back to the stash token
so paragraphs above don't bleed into the <pre> block.
20 regression tests in tests/test_issue1438_fence_anchoring.py cover:
- Cygnus's exact repro from Discord (May 1 2026)
- Inline ``` mid-paragraph (must not open fence)
- Partial/streaming fence with no close (must not eat content)
- Empty fences with and without language tag
- 3-space indented fences (allowed) vs 4-space (not a fence)
- Multiple adjacent blocks
- Bold/italic/inline-code surviving after a fence
- Source-level guards on all 3 patched sites + lead-prefix invariant
Empirical browser verification (live JS, on bug repro):
Before fix: </code></pre>[^\n]<em>|%%[ \t]</em>... ← truncated, italic leak
After fix: <pre><code>...```[^\n]*|%%...</code></pre> ← intact, regex preserved
Tests: 3678 passed (+20 from new test file, was 3658), 0 failures.
Reported-By: Cygnus (Discord)
Relayed-By: @AvidFuturist
Closes#1438
