fix(editor): preserve selection across view modes

[murongg]

Summary

• preserve CodeMirror selections and focus when switching between Preview, Source, and Split modes
• remeasure the revealed editor surface so long documents do not appear blank after a mode switch
• preload the source editor chunk during idle time after the visual editor is ready
• reuse the same module promise for preloading and lazy rendering, with a source-shaped loading placeholder as fallback

Why

Switching modes recreated the source editor at offset 0 and could leave the visual editor unfocused or measured while hidden. This lost the active cursor position and could make the editing surface appear blank. The first switch also waited for the source editor chunk even when the app had already become idle.

Validation

• pnpm --filter @markra/app exec vitest run — 130 test files and 1493 tests passed
• pnpm --filter @markra/desktop build — production build and vendor chunk verification passed
• manual browser QA with an 80-section document across Preview → Source → Split → Preview — cursor continuity, focus restoration, visible rendering, and zero console errors confirmed

Risk

• preloading starts only after the active visual editor is ready and uses a cancellable idle callback or delayed timeout fallback
• selection offsets are clamped to the current document length before restoration
• no hidden source editor instance, persisted-data change, or configuration migration

Closes #649

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Project	Deployment	Actions	Updated (UTC)
markra-web	Ready	Preview	Aug 8, 2026 12:23am