Browser screenshot capture reused composer prefill, which replaced local draft text and remained queued after the composer consumed it. The fix keeps rewind-style prefill as replacement while letting Browser screenshots append as one-shot composer attachments.
Constraint: Screenshot events can arrive while the chat composer is unmounted during Settings navigation.
Rejected: Store browser screenshots only in composerDraft | mounted composers would not update immediately.
Confidence: high
Scope-risk: narrow
Directive: Keep Browser screenshot capture on append mode; replacement prefill is for rewind-style composer restoration.
Tested: bun run test -- ChatInput.test.tsx previewEvents.test.ts --run
Tested: bun run check:desktop
Not-tested: Live Windows desktop/Tauri screenshot click
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