fix(canvas): prevent coordinate drift and redundant saves on navigation#11
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Polygon points were drifting by 1px per save/load cycle when navigating between images. Two bugs combined to cause this: serializer used only 3 decimal places (insufficient for sub-pixel accuracy at common resolutions), and the deserializer used truncation instead of rounding. At 4x zoom the drift was visible as a 4px jump per navigation cycle, stabilizing after 2-3 passes. Also removes redundant AutoSaveCurrentImage call from LoadImageAtIndex. The PolygonsChanged signal already triggers a synchronous save before navigation can occur, so the explicit call was dead code. Undo and Redo now check CanUndo/CanRedo before emitting PolygonsChanged, and Paste no longer emits the signal when the clipboard is empty. - Increase serializer precision from 3 to 6 decimal places - Replace static_cast<int> truncation with std::round in deserializer - Guard Undo/Redo emission behind CanUndo/CanRedo checks - Guard Paste emission behind successful paste check - Remove AutoSaveCurrentImage from LoadImageAtIndex (dead code) - Add RoundTripNonDivisibleCoordinates and RoundTripStableAcrossMultipleCycles tests - Fix KeyPressCtrlZCallsUndo test and add missing Undo/Redo/Paste signal tests
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Summary
static_cast<int>) instead of rounding in the deserializer. At 4x zoom this appeared as a 4px jump per navigation cycle, stabilizing after 2-3 passes.AutoSaveCurrentImagewas called unconditionally inLoadImageAtIndex, but thePolygonsChangedsignal already triggers a synchronous save before navigation occurs — making the explicit call dead code.PolygonsChangedemissions:Undo/Redoemitted the signal even when there was nothing to undo/redo;Pasteemitted it even when the clipboard was empty.Changes
static_cast<int>truncation withstd::roundin deserializerUndo/Redosignal emission behindCanUndo/CanRedochecksPastesignal emission behind successful paste result checkAutoSaveCurrentImagecall fromLoadImageAtIndex(dead code)Test plan
KeyPressCtrlZCallsUndotest (was incorrectly passing withCanUndo=false)UndoSkippedWhenCanUndoFalseRedoSkippedWhenCanRedoFalsePasteEmitsPolygonsChangedOnSuccessPasteDoesNotEmitPolygonsChangedWhenClipboardEmptyRoundTripNonDivisibleCoordinates— verifies exact round-trip for coords that drift with 3dec+truncation (e.g. 105/640)RoundTripStableAcrossMultipleCycles— simulates 5 navigation cycles, verifies zero drift