fix(editor): preserve source scroll position while editing

[murongg]

Summary

• preserve the external source-editor viewport across CodeMirror input measurements
• only scroll after restoration when the updated cursor is actually outside the viewport
• cover regular typing, IME composition, deletion, off-screen cursor reveal, and stale navigation snapshots

Why

Source mode delegates vertical scrolling to .paper-scroll while CodeMirror virtualizes the long document inside .cm-scroller. The first edit after selecting a visible line could trigger a delayed measurement correction and move that line to the top of the viewport.

Validation

• pnpm --filter @markra/editor test — 40 files, 531 tests passed
• pnpm --filter @markra/app test — 134 files, 1555 tests passed
• pnpm --filter @markra/editor build — passed
• pnpm --filter @markra/app build — passed
• Browser QA with a synthetic 600-line document: typing and Backspace on a middle line both kept scrollTop at 7268.5 and the line offset at 305.6875px

Risk

• Scoped to source mode editors that use an ancestor scroll container
• Typewriter mode bypasses the preservation behavior so its intentional centering remains unchanged

[vercel]

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Project	Deployment	Actions	Updated
markra-web	Ready	Preview	Sep 4, 2026 2:44pm UTC