fix(web): hide YAML frontmatter in Markdown file previews#23914

Problem

Opening a Markdown file with leading YAML frontmatter from Files renders the metadata as a horizontal rule and a large setext heading. The document should start at its body heading.

Change

Mask complete leading YAML frontmatter in web/desktop file and attachment previews. Preserve every source offset and line break so task checkbox edits still target the original file. Source view and saved contents retain the metadata. Recognize BOM-prefixed files and LF, CRLF, or CR line endings.

Scope and approval

Fixes #23704

Triaged bug via the automated issue flow. Reproduced on current main in the real web app with the reporter's exact fixture. Mobile uses a separate renderer and is outside this reported web/desktop fix.

Verification

• vp test run src/components/files/FilePreviewPanel.test.ts from apps/web: 17 passed. BOM and CR-only cases failed before the follow-up fix.
• Targeted lint and formatting passed; web tsc --noEmit -p apps/web passed.
• Real app: Files preview and draft attachment preview at 1440 and 390 CSS pixels, light and dark. Source/render toggles and task checkbox persistence checked with paired recordings and byte comparisons.

Evidence

Evidence index. Independent proof audit reproduced F1-after pixel-identically and verified all 36 artifact hashes and all eight diff pairs.

Same reporter bytes, route /draft/52c1b0b1-6ff3-4fb8-990a-967b991c033f, layout and theme within each still pair. Files panel maximized, file explorer collapsed. Chrome on Linux; themes follow browser prefers-color-scheme. No device emulation; default font scaling. CSS viewports 1440×900 and 390×900, zoom 100%, DPR 2. Browser tool reduces desktop screenshots to 1280×800; narrow screenshots are 780×1800. These are responsive web captures, not native mobile.

Diff0 = per-channel absolute difference amplified 8×, no threshold. DiffT = magenta where maximum RGB channel difference >16/255, black elsewhere. No alignment, crops, masks, or exclusions. Pixel counts include app chrome. Artifact SHA-256 is shown beneath every link. Before code: 51273020db; after code: 373457aebc (subsequent commits only add fixtures).

F1–F4: Files rendered preview passes: one body heading, no metadata/rule/setext artifact or masked gap. At 390 the body wraps. D1–D4: supplementary draft Blob attachment pairs show the same corrected body using AttachmentFilePreview; these do not establish sent asset fetching.

I1 task interaction, 1440×900 light. Same companion bytes and ordered actions. Timing and sidebar/composer framing differ between recordings; source syntax highlighting loads asynchronously. Recordings are normal speed. Approximate timestamps:

• Before (14.109 s): 0–7 rendered → source with YAML intact; 7–9 rendered → first task checked → source; 9–11 source → rendered → uncheck; 11–14 close/reopen, explorer expanded.
• After (19.158 s): 0–6 rendered → source with YAML intact; 6–9 rendered → first task checked → source; 9–16 source → rendered → uncheck; 16–19 close/reopen, explorer expanded.
• Both versions: disk-byte assertion after checking equals original reporter bytes plus - [x] first task and - [ ] second task; after unchecking, byte comparison equals seeded original. No YAML or second-task mutation.

Draft interaction, 390×900 dark: before 9.230 s; after 7.727 s. At 0 s switch rendered → source; approximately 1–6 s inspect original YAML, closing fence, body heading; approximately 6–8 s return rendered → close → reopen. Before shows metadata in rendered view; after hides it. No task editing in attachment preview. Source syntax highlighting briefly loads, then displays original metadata.

L1 passes: real composer file intake, draft chip, rendered/source/close, exact reporter fixture. C1 passes in Files and draft: Control heading, Paragraph, body horizontal rule, Body after rule, checkbox retained. C2 visual proof is not claimed: formatting normalized its opener to ***; the focused unit test covers true unterminated frontmatter.

Unverified: A1–A4 and I2 sent attachment. No persisted sent fixture was available in this tab's isolated draft state. Sending via composer would invoke a provider; no provider was invoked and no synthetic sent state was injected. Draft proof is supplementary, not a substitute for these matrix rows. Desktop shares web components but was not separately launched. Native mobile, relay/tunnel, other providers, chat Markdown and other preview formats were not exercised; mobile renderer remains unchanged.

Visible differences: the frontmatter rule and metadata heading disappear, and the document body moves up to the beginning of the preview. Source metadata and task edits remain intact.

Reproduce with node apps/web/test/fixtures/frontmatter-preview/seed.mjs <workspace-root>, then open frontmatter-repro.md from Files or attach it in the composer. The committed seed recreates the exact reporter fixture and task companion.

Models: Claude Opus 5.5 (Claude Code harness) and GPT-6.1-Sol (Codex harness).