feat(canvas): smart-align guides during line endpoint resize#50
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Line endpoint resize previously bypassed Konva's Transformer (it uses custom <Rect> handles in LineObject), so the Stage-level object-snap that fires for normal drag never kicked in — endpoints could only free-form-snap to the grid. Plumb getOthersSnapshot / labelRect / setGuides through KonvaObjectProps so LineObject can run its own snap during endpoint drag: - snapEndpoint(): zero-size rect snap via computeSnap against neighbour edges + label rect, in stage coords. Local↔stage conversion via the parent group's absoluteTransform keeps the snap correct under rotated view (viewRotation 90°/270°). Snap is skipped when Shift is held (axis constraint wins) and when grid-snap is on (mutual exclusion with the existing drag-time snap, mirroring the rest of the app). - snapshot is cached at drag-start to avoid re-querying every other node's clientRect per frame; cleared on drag-end. - selectionHandlers helper applied to BarcodeObject's remaining unfixed Group node (caught while threading new KonvaObjectProps imports). Open trade-off: snap picks the nearest neighbour edge by raw distance. For aligning a line endpoint to the *matching* edge of another line (bottom-to-bottom etc.) the user may need to drag past the near edge. A side-aware heuristic (which edge to prefer when dragging in a given direction) is a follow-up — orthogonal to this branch's scope.
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Code Review
This pull request implements object-snapping for line endpoints, allowing lines to snap to the edges of other shapes and labels during resizing. It introduces new hooks in the KonvaObjectProps interface and a snap-guide pipeline in LineObject.tsx that handles coordinate conversions for rotated views. Feedback was provided regarding a performance optimization in LabelCanvas.tsx, where defining getOthersSnapshot inside a loop causes unnecessary function re-creations on every render.
Reviewer-spotted (gemini-code-assist on PR #50): defining the snapshot closure inside objects.map allocated a fresh function for every object on every LabelCanvas render — and LabelCanvas re-renders on every setGuides call during a drag. Lift to a single useCallback-stable function that takes excludeId. The KonvaObjectProps signature changes from () => SnapRect[] to (excludeId) => SnapRect[]; LineObject's lazy snapshot call now passes obj.id.
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