Cutting-layer produces empty “island” when following the outer plan boundary

Summary

When the cutting-layer line exactly overlaps the outer boundary of a plan polygon, the split algorithm creates an empty or invalid “island” polygon. Instead of trimming the outer ring, the algorithm leaves behind the outer boundary.

Steps to reproduce

1. Load a plan.
2. Draw or load a cutting-layer feature whose geometry is exactly coincident with a segment of that outer boundary (Planobjekt).
3. Run the split tool.
4. Inspect the resulting Innen/Außen layers at the overlap location.

What is the current behavior?

The split operation produces a "hole" in the Außen plan with the coincident boundary segment as the new plan's outer boundary.

What is the expected correct behavior?

When the cutting line coincides with the outer boundary, that boundary segment should be trimmed or removed. No "hole" should be produced.

Relevant logs and/or screenshots

Possible fixes

• Detect boundary-coincident cuts and treat them as boundary trimming instead of polygon splitting.
• After splitting, filter out zero-area or invalid polygons.