fix(mobile): render Markdown backslash escapes - #8925

…8940)

Follow-up to [#8925](#8925), from a
[Codex
finding](#8925 (comment))
there.

After #8925, message Markdown arrives with backslash escapes shielded,
but imeta is keyed by plain URLs. Two lookups used the raw,
still-escaped destination:

- **Bubble segmentation** (`bubble_segments.dart`): an attachment link
such as `[voice.mp4](https://…/voice\_note.mp4)` missed its imeta entry
and stayed inside the text bubble, even though the link builder rendered
it as an attachment. The fix unshields the URL before the lookup.
- **Trailing image gallery** (`extractTrailingImageGallery` in
`message_gallery.dart`): with two or more trailing images, an escaped
destination kept its backslash, so the carousel loaded the wrong URL and
lost the imeta alt text. The fix unescapes the destination the same way
as the label beside it. Its callers in `message_media_avatar.dart` and
`message_skeleton_body.dart` get the fix too. This gap was already on
main.

### Validation

- The existing `attachments stay outside bubbles and text order is
preserved` journey now uses an escaped destination with a plain imeta
URL. The existing carousel journey now has one escaped image destination
and taps it by its plain URL. Each fails with its fix reverted.
- `flutter analyze` is clean and the full mobile suite passes (4151
tests).
- Limitation: no simulator run. Both changes are key lookups, and the
widget journeys exercise the rendered layout. The full-app run for the
surrounding escape handling is recorded in #8925.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Signed-off-by: Tom Brow <[email protected]>
Co-authored-by: Tom Brow <[email protected]>
Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@buzz.block.builderlab.xyz>
Co-authored-by: Claude Opus 5.5 <[email protected]>

Main now shields backslash escapes before rendering (#8925), so a
trailing `\]` reaches the marker rule as text, never as display math.
Drop that own-line rule; an escaped bracket now takes the inline
marker. The thread edit-reload test from #8870 now expects the
marker joined to the reply text.

Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>
Co-Authored-By: Claude Opus 5.5 <[email protected]>
Signed-off-by: Logan Johnson <[email protected]>

Main now shields backslash escapes before rendering (#8925), so a
trailing `\]` reaches the marker rule as text, never as display math.
Drop that own-line rule; an escaped bracket now takes the inline
marker. The thread edit-reload test from #8870 now expects the
marker joined to the reply text.

Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>
Co-Authored-By: Claude Opus 5.5 <[email protected]>
Signed-off-by: Logan Johnson <[email protected]>

Main now shields backslash escapes before rendering (#8925), so a
trailing `\]` reaches the marker rule as text, never as display math.
Drop that own-line rule; an escaped bracket now takes the inline
marker. The thread edit-reload test from #8870 now expects the
marker joined to the reply text.

Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>
Co-Authored-By: Claude Opus 5.5 <[email protected]>
Signed-off-by: Logan Johnson <[email protected]>

A backslash inside an angle autolink is literal in CommonMark, but
shielding turned `\>` into a stand-in, so <https://e.com/a\> lost its
closing bracket. Leave angle autolinks untouched (doubling only authored
markers).

gpt_markdown 1.3.4 now restores escapes in multi-line strikethrough, so

Signed-off-by: Samin Yeasar Sohag <[email protected]>
block#8925's known-limitation row expects # instead of its fullwidth form.

A backslash inside an angle autolink is literal in CommonMark, but
shielding turned `\>` into a stand-in, so <https://e.com/a\> lost its
closing bracket. Leave angle autolinks untouched (doubling only authored
markers).

gpt_markdown 1.3.4 now restores escapes in multi-line strikethrough, so

Signed-off-by: Samin Yeasar Sohag <[email protected]>
block#8925's known-limitation row expects # instead of its fullwidth form.

Main now shields backslash escapes before rendering (#8925), so a
trailing `\]` reaches the marker rule as text, never as display math.
Drop that own-line rule; an escaped bracket now takes the inline
marker. The thread edit-reload test from #8870 now expects the
marker joined to the reply text.

Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>
Co-Authored-By: Claude Opus 5.5 <[email protected]>
Signed-off-by: Logan Johnson <[email protected]>

Main now shields backslash escapes before rendering (#8925), so a
trailing `\]` reaches the marker rule as text, never as display math.
Drop that own-line rule; an escaped bracket now takes the inline
marker. The thread edit-reload test from #8870 now expects the
marker joined to the reply text.

Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>
Co-Authored-By: Claude Opus 5.5 <[email protected]>
Signed-off-by: Logan Johnson <[email protected]>

Message Markdown on mobile moves from gpt_markdown 1.2.1's legacy regex
pipeline to the CommonMark parser that 1.3 added ("plusparse"). Before
this, passing `inlineComponents` (now deprecated, removed in 2.0.0) kept
every message on the legacy pipeline.

### What changes for users

Rendering moves closer to desktop (react-markdown + remark-gfm):

- `_italic_` and `__bold__` work, backslash escapes and HTML entities
decode, and `[a [b] c](url)` labels are correct.
- Inline code follows CommonMark pairing: `"```npm install```"` in a
sentence is inline code, and four-backtick fences work.
- Setext headings, reference links, footnotes, `~~~` fences, `1)` lists
and CommonMark list/quote continuation render.
- HTML comments are hidden, as on desktop.
- `\[`, `\]`, `\(` and `\)` stay literal, as on desktop. Plusparse
hard-codes them as LaTeX delimiters, so they're written as entities
before rendering.
- Side effect: an unfenced `__init__` now renders as bold "init", and a
line over `---` becomes a heading, same as desktop.

Device benchmark (iPhone SE, profile build, 600 synthetic messages, 3
launches per build): scrolling spends 9% less time building, and frames
with more than 8 ms of build drop from 63 to 22. Jumping onto unbuilt
messages is 11% cheaper, and raster time is unchanged.

App size: the release app grows about 4.6 MB uncompressed (80.6 to 85.2
MB), mostly Dart code from the new math and highlighting packages.

### Before and after

Synthetic messages that reproduce differences found in real channel
history, rendered through `MessageContent` on main and on this branch:


![01-escaped-brackets-in-code](https://raw.githubusercontent.com/block/buzz/49600d2f9b87c2c41fd1bcac450efd345de0d0db/pr-8945--01-escaped-brackets-in-code.png)


![02-inline-triple-backticks](https://raw.githubusercontent.com/block/buzz/49600d2f9b87c2c41fd1bcac450efd345de0d0db/pr-8945--02-inline-triple-backticks.png)


![03-four-backtick-fence](https://raw.githubusercontent.com/block/buzz/49600d2f9b87c2c41fd1bcac450efd345de0d0db/pr-8945--03-four-backtick-fence.png)


![04-entities](https://raw.githubusercontent.com/block/buzz/49600d2f9b87c2c41fd1bcac450efd345de0d0db/pr-8945--04-entities.png)


![05-stars-and-underscores](https://raw.githubusercontent.com/block/buzz/49600d2f9b87c2c41fd1bcac450efd345de0d0db/pr-8945--05-stars-and-underscores.png)


![06-ordered-list-paren](https://raw.githubusercontent.com/block/buzz/49600d2f9b87c2c41fd1bcac450efd345de0d0db/pr-8945--06-ordered-list-paren.png)


![07-setext-heading](https://raw.githubusercontent.com/block/buzz/49600d2f9b87c2c41fd1bcac450efd345de0d0db/pr-8945--07-setext-heading.png)


![08-dunder](https://raw.githubusercontent.com/block/buzz/49600d2f9b87c2c41fd1bcac450efd345de0d0db/pr-8945--08-dunder.png)

### Demo

A scroll through synthetic messages covering common formatting
(headings, lists, quotes, links, highlighted code, a table, a task list,
escapes), recorded on an iPhone 17 Pro simulator running this branch:


https://github.com/user-attachments/assets/bdc9680c-d20b-41ec-8e1a-a5b51c602f9d

### How

- Mentions, custom emoji and `#channel` tokens are `InlinePattern`s.
Links go through `inlineLinkBuilder` into the existing `_buildLink`, so
chips, media links and taps are unchanged.
- Removed workarounds the parser makes unnecessary:
  - the render-path escape shielding from #8925
- the `_MarkdownEscapeMd`, `_MarkdownLabelEntityMd` and `_MessageLinkMd`
components
  - the zero-width prefix before a leading `#channel`

Plain-text previews and gallery helpers keep their own escape handling.
`normalizeBareLinks` stays because attachment and gallery detection
depend on its output. Bubble segmentation unescapes a link destination
itself before its imeta lookup, since content no longer arrives
shielded.
- Kept desktop parity where plusparse differs from GFM:
- a table typed directly under a caption line gets a blank line inserted
before it, since plusparse can't start a table mid-paragraph
- `> [!NOTE]` quotes render as quotes (`alertBuilder` returns
`asBlockQuote()`), since desktop has no alerts
- `mapMarkdownProse` now ends an unclosed backtick run at its paragraph.
Before, it could pair with a run in a later block and flip code and
prose for the rest of the message.
- Synthesized autolink labels escape backslashes instead of using
`\`, which the parser leaves undecoded in a URL-shaped label.
- Repo/PR/issue chips keep their own semantics node.
- Forum card previews clip at their 120 px cap instead of overflowing:
blocks now lay out as a column.
- Pinned to exactly 1.3.4. 1.4.0 renders HTML by default and themes
through the separate `material_ui` package, which would need a theme
bridge.

### Validation

- Full mobile suite passes and analyze is clean at 78b9c1a.
- All of #8925's and #8940's backslash-escape cases pass on the new
parser, including the strike-across-lines case that used to be a known
limitation.
- Real-message diff: 2,259 recent messages from 13 busy channels,
rendered through `MessageContent` on main and on this branch. 29
differed in text or styling. On inline and fenced code, the branch
matched desktop's parser (remark-gfm) in 23 of them and main in 7. The
review found the three parity gaps above, now fixed. The rest are
CommonMark behavior that desktop shares.
- Added tests, each confirmed to fail without its fix:
  - a leading `#channel` renders as a link, not a heading
  - a table under its caption renders as a table
  - an alert-marker quote stays a quote
  - an unclosed backtick run doesn't flip later code
- the transcript tests detect a rendered link by its style; the old
`LinkButton` check passed even with autolink on
- Automated simulator run (iPhone 17 Pro, iOS 27, debug build, signed in
to a Builderlab community): channel list, two channels with history
scrolled back several pages (mentions, link previews, thread summaries,
images), an agent Solo chat with long agent messages, and the inbox. No
rendering errors or layout breaks; the Solo chat screen also matched
main's build side by side.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Signed-off-by: Tom Brow <[email protected]>
Co-authored-by: Tom Brow <[email protected]>
Co-authored-by: studio <dc2fc481343672df31a689840c09299972e704dac592e0da6f18486646a7ee3e@buzz.block.builderlab.xyz>
Co-authored-by: Claude Opus 5.5 <[email protected]>