[pythonicode]

Summary

Fixes the touch-screen pinch zoom where the point being zoomed drifted away from the fingers.

Stacked on #3 (feat/responsive-layout). Its base is that branch, so the diff shows only this fix.

Cause

• Competing stage drag. On the Pan tool, the canvas is a Konva draggable stage. A one-finger drag could still be running when a second finger landed, and the drag and the pinch then both moved the stage.
• Anchor maths. The pinch added the finger movement a second time, so the anchor drifted each frame. The fallback path also mixed layer and map coordinates, which was wrong for rotated maps.

Fix

• A pinch stops any running stage drag and restores it afterwards (onGestureStart / onGestureEnd in usePinchZoom).
• The pinch starts from the stage's live transform, not the stored viewport.
• The anchor step is one pure function, pinchViewport, which keeps the stage point under the previous centre under the new centre. It has unit tests for zoom, pan, clamping and rotation.

Testing

• bun run test (28 pass, 4 new), bun run lint, bun run build pass.
• In the browser, synthetic two-finger pinches on the phone layout show zero drift at rotations 0°, 30°, 45° and 90°, on both Select and Pan tools, with and without the stage already moved. A drag running at pinch start is stopped.
• Not verified on a real phone. Synthetic pointer events can't reproduce the real finger timing. The on-device check is the one that matters.

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated
analysis	Ready	Preview	Oct 14, 2026 9:16am UTC