fix(editor): stabilize math preview scrolling

[murongg]

Summary

• stop rebuilding all math decorations during viewport-only scroll updates
• replace multiline formulas with a single state-backed CodeMirror block replacement
• remove the .cm-line display: none layout workaround
• cache rendered KaTeX across selection, focus, and Vim reveal-only updates
• estimate offscreen formula height from the original source footprint
• keep math focus synchronization out of active IME composition

Why

The issue attachment has roughly 929 math ranges. Viewport updates previously rescanned and rerendered all of them, while multiline formulas hid individual editor lines outside CodeMirror’s block height model. Together these could make large scroll jumps expensive and leave WebKit with stale document geometry.

Moving block decorations into editor state fixed the height model, but the first CI run also exposed a focus-sync transaction racing controlled Chinese IME input. Focus changes are now applied synchronously before composition, deferred while composition owns the editable DOM, and reconciled after composition ends.

Validation

• pnpm --filter @markra/app test — 1,494 tests passed, including the controlled IME/image regression
• pnpm --filter @markra/editor test — 450 tests passed
• pnpm --filter @markra/app build and typecheck:test — passed
• pnpm --filter @markra/editor build and typecheck:test — passed
• pnpm --filter @markra/desktop build — passed, including vendor chunk verification
• clean Chromium session with the issue attachment — a 14,000px jump reached section 7.2 with 15 visible lines, 4 display formulas, 0 hidden math lines, stable 160079px height at 150ms/850ms, and no error toast

Risk

• Tauri dev launched successfully, but the Mac was locked so the WebKit window could not be operated for the final smoke test. The PR remains draft until that target-specific check is completed.
• Parallel local stress runs exposed existing deferred-parser timing flakes in unrelated preview tests. The affected files passed in isolation, and the sequential editor suite passed 450/450.

Refs #654

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Project	Deployment	Actions	Updated (UTC)
markra-web	Ready	Preview	Aug 8, 2026 5:16am