Selection screenshots were drawing the marker after html2canvas finished by
mapping DOM coordinates onto a separate canvas. That leaves room for drift when
html2canvas renders at a different output size, the viewport is resized, or the
thumbnail is compressed. The marker is now a fixed DOM overlay captured with the
page itself, so the chosen element and its marker share the same viewport,
layout, crop, and scale path.
Constraint: No issue number was provided for this task.
Constraint: Preserve the existing selection event payload and preview bridge protocol.
Rejected: Keep canvas post-processing with more scale math | it still maintains a second coordinate system beside the rendered page.
Rejected: Keep the unused annotate module | it would preserve the stale path that caused this class of drift.
Confidence: high
Scope-risk: narrow
Directive: Do not reintroduce canvas-side selection marker math unless it is proven against resized and scaled viewport captures.
Tested: cd desktop && bun run test --run src/preview-agent/screenshot.test.ts src/preview-agent/picker.test.ts src/preview-agent/metadata.test.ts src/preview-agent/protocol.test.ts src/preview-agent/bridge.test.ts src/preview-agent/editBubble.test.ts
Tested: cd desktop && bun run lint
Tested: cd desktop && bun run build:preview-agent
Tested: bun run check:desktop
Tested: git diff --check
Not-tested: Real Tauri webview visual smoke.
Element selection from the local preview should preserve the visual target
the user picked and send the confirmed request directly to chat. The
screenshot now captures the visible viewport before drawing the marker, and
the selection event sends a model-facing prompt while the chat UI shows only
the annotated image and compact element label.
Constraint: No issue number was provided for this task.
Constraint: Keep the preview-agent protocol shape unchanged.
Rejected: Prefill the composer with the selection note | it keeps the old extra confirmation step and exposes implementation text in the input.
Rejected: Full-document annotation coordinates | body-relative captures drift from the viewport selection users see.
Confidence: high
Scope-risk: moderate
Directive: Do not route confirmed preview selections back through composer prefill without rechecking the Codex-style direct-send flow.
Tested: cd desktop && bun run test --run src/preview-agent/screenshot.test.ts src/lib/previewEvents.test.ts src/lib/selectionComposer.test.ts src/components/chat/AttachmentGallery.test.tsx src/stores/chatStore.test.ts
Tested: cd desktop && bun run lint
Tested: cd desktop && bun run build:preview-agent
Tested: bun run check:desktop
Tested: git diff --check
Not-tested: Real Tauri webview click-through smoke; in-app Browser automation timed out while local Vite HTTP smoke returned 200.
Wire createPicker into the preview-agent IIFE: enter/exit picker via host messages, bind DOM mousemove/click (capture phase) to picker hover/select, and on click emit a selection event carrying ElementMetadata + annotated screenshot. Route the selection event in previewEvents.ts through buildSelectionComposerText into queueComposerPrefill with the screenshot attachment.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
