fix(composer): scope multiline block formatting

[tellaho]

Category: fix
User Impact: Composer block formatting now applies to the intended line or selection without collapsing multiline content.

Problem: Block formatting from a Shift+Enter line could convert the entire draft, selected visual lines could collapse into one list item, and code conversion could lose line breaks. Solution: Scope caret formatting to its hard-break-delimited line and normalize explicit selections for the destination block type while preserving neighboring content and visual line boundaries.

File changes

desktop/src/features/messages/lib/selectionBlockFormatting.ts
Scopes collapsed-caret block actions to the active visual line and normalizes multiline selections for lists and code blocks.

desktop/src/features/messages/lib/selectionBlockFormatting.test.mjs
Adds unit coverage for caret-line isolation across line positions and selection directions.

desktop/src/features/messages/ui/FormattingToolbar.tsx
Routes list, quote, and code-block actions through the selection-aware formatting transaction.

desktop/tests/e2e/composer-selection-formatting.spec.ts
Covers caret-only formatting, multiline list conversion, list-to-code conversion, preserved hard breaks, Markdown output, and backward selections.

Reproduction steps

1. In the desktop composer, enter several lines using Shift+Enter and place the caret on one line.
2. Apply a bullet list, ordered list, quote, or code block; only the caret line should change.
3. Select several Shift+Enter lines and apply a list; each visual line should become its own item.
4. Select several list items and apply Code block; they should become one multiline code block while unselected neighbors remain intact.
5. Select several Shift+Enter lines and apply Code block; each line break should remain visible.

Screenshots/Demos

Expected multiline code-block result: https://buzz.block.builderlab.xyz/media/d2e2668093af3b67d896a32e9799daccd236da9fc9e24ec56ddb4ebf7d01dd96.png

[wpfleger96]

🤖 Thanks for the fix — the caret-line isolation is nicely done (first/middle/last/empty trailing lines all covered, backward selections restored, single undo unit) and the serialized-Markdown e2e assertions are the right end-to-end check.

That said, we found two selection shapes where unselected content gets structurally reformatted, plus a branch that I think defeats the toolbar's own WKWebView selection workaround, so requesting changes. Details inline on the relevant lines.

A couple of non-blocking notes while you're in here:

• selectionBlockFormatting.test.mjs only covers isolateSelectionForBlockFormatting — splitSelectedLinesForListFormatting and mergeSelectedTextblocksIntoCodeBlock are e2e-only. They're pure transaction logic so unit cases should be cheap to add.
• textblockTextForCode() drops any leaf node that doesn't implement leafText, so mentions/custom emoji could silently disappear when converted to code. Probably fine as a contract, but worth a unit case or a comment saying it's intentional.

[tellaho]

@wpfleger96 We also closed out both non-blocking notes from your review:

• 21987a8d2fb772be2580933f53762b8f6dc0d925 preserves meaningful inline-atom text during code-block conversion, documents the fallback semantics, and adds focused unit coverage for schema leafText, mention labels, custom-emoji shortcodes, hard breaks, and safe dropping of unknown leaves.
• fb91387616f93e38eda66f7e6c410f255614195f tightens the partial-list Playwright regression to assert exact top-level structure, item contents, and preserved neighbors for code, bullet, ordered, and quote outcomes.

Validated with 3,731 desktop unit tests, typecheck/check, and focused composer Playwright 24/24.

— AI-generated response on behalf of Taylor Ho