Find in the markdown preview and in any diff

[afitzgerald]

What

⌘F / Ctrl+F (rebindable, in the Edit menu) dispatches on whatever the diff pane is showing:

• Rendered markdown preview — the one view with no editor behind it, so it gets its own find bar: case-insensitive substring search, n/m counter, Enter / ⇧Enter to cycle with wrap, Esc to close. Matches are painted with the CSS Custom Highlight API (Ranges, no DOM mutation), so the rendered document and its diff tinting stay exactly as rendered, and clearing is one delete() per highlight. Re-runs whenever the pane re-renders (mode switch, file switch, live edit) since the old Ranges point at nodes that are gone.
• Any diff / conflict editor — hands over to Monaco's own find widget. The global keydown handler swallows the key before Monaco sees it, so the handover is explicit.

Mermaid fix (separate commit)

Several of mermaid's bundled deps ship classic UMD wrappers that test typeof define === 'function' && define.amd first. Monaco's AMD loader already owns a global define by the time markdown.ts lazy-loads renderer/mermaid.js, so they registered as anonymous modules and the bundle died with Can only have one anonymous define call per script file — every mermaid fence fell back to raw source in the shipped app, not just in tests. The bundle is now wrapped in (function(define){ … })().

UI test harness

Three things kept node test/ui.test.js from running at all:

• the navigator.platform pin (which gives the suite Mod = Ctrl semantics) was defined on the instance, where current Chromium makes it non-configurable — it threw Cannot redefine property and silently never applied. Now on Navigator.prototype.
• #diff-count renders "2 differences (+2 −2)", but two assertions still compared against the bare "2 differences".
• the Chromium path fell back to a sandbox-only location; it now uses Playwright's own downloaded browser when that path is absent (DIFFIER_CHROMIUM still overrides).

Known limitation

Markdown matches don't span inline element boundaries — a query crossing **bold** won't hit. Flagged in a ponytail: comment; needs a concatenated-text offset map if it ever matters.

Testing

yarn typecheck, yarn test, and node test/ui.test.js all pass (7 new UI assertions cover the Monaco handover, the bar opening + focus, the counter, Enter/⇧Enter wrap, the no-match state, and Esc clearing the highlights).