editor: conflict compare unsaved pane still opens scrolled to bottom

[drawmeanelephant]

Summary

Retest of #975 / PRs #981–#982 on main (23ccd757) found the conflict compare unsaved pane still opens scrolled near the bottom. The disk pane correctly starts at scrollTop === 0.

Observed: #unsaved-version.scrollTop ≈ 6490 with scrollHeight ≈ 6696 on first open after dirty + external edit. Left column shows the typed tail; right column shows the file head — easy to misread as a large structural diff.

Repro

1. Launch boris-editor against the repo.
2. Open a reasonably long content file; dirty the buffer near the end.
3. Change the same file on disk externally so the conflict dialog opens.
4. Inspect #unsaved-version vs #disk-version scrollTop (or just look at the two columns).

Expected

Both compare panes start at the top when the dialog opens (or stay scroll-synced).

Actual

Disk pane top; unsaved pane near end. $effect in editor/ui/src/dialogs/ConflictDialog.svelte already sets scrollTop = 0 when content flags change, but deps are buffer.content / conflict content / deleted flag — not the textarea element refs. showModal focusing #unsaved-version can jump caret/scroll to the end after that reset.

Suggestion

Reset after mount/focus: include textarea refs in the effect, or await tick() after showModal and force both panes to 0 again. Add a Playwright assertion that both #unsaved-version and #disk-version have scrollTop === 0 on first open.

Evidence

Hands-on retest 2026-09-13; report /workspace/boris-editor-retest.md (local). Related closed umbrella: #975.