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… merge vertex outside of the shape The test case provided in issue 963 triggered the error recorvery path which sorts all acitve edges. After the sort there is a fixup pass for merge vertices which swaps them with the previous edge if the merge vertex is outside of the shape (working under the over-simplified assumption that if the merge vertex is outside, then the previous edge is the in-out boundary). However if the previous edge was the result of fusing two overlapping edges with opposite windings, that edge would have zero-winding and would therefore not be the boundary, leaving the merge vertex outside of the shape, still. The fix is to search for the closest edge that is inside of the shape instead of blindly picking the previous one. If none is found on the left, then search on the right side. It is still technically possible that all active edges have zero-winding, which means that there is nowhere to put the merge vertex. That case seems pretty difficult to construct, and there isn't much that can be done in this case.
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… merge vertex outside of the shape
The test case provided in issue 963 triggered the error recorvery path which sorts all acitve edges. After the sort there is a fixup pass for merge vertices which swaps them with the previous edge if the merge vertex is outside of the shape (working under the over-simplified assumption that if the merge vertex is outside, then the previous edge is the in-out boundary). However if the previous edge was the result of fusing two overlapping edges with opposite windings, that edge would have zero-winding and would therefore not be the boundary, leaving the merge vertex outside of the shape, still. The fix is to search for the closest edge that is inside of the shape instead of blindly picking the previous one. If none is found on the left, then search on the right side. It is still technically possible that all active edges have zero-winding, which means that there is nowhere to put the merge vertex. That case seems pretty difficult to construct, and there isn't much that can be done in this case.
Fixes #963.