Merging zones doesn't always work

Description

This issue was already reported and marked as fixed: #13628 (closed)

There is another similar issue open but they specify only for multiple zones: #11492 (closed)

The issue is the same as in the first link. Sometimes the overlapping zones can't be merged due to error "Zones have insufficient overlap for merging". No matter how big an overlap is, the error is always there. Creating new zones and merging them with problematic zones works.

Unfortunately, I am unable to share the project file.

I have found a workaround. Select the zones and:

  • select the problematic zones
  • create from selection -> create lines from selection (possibly something else would also work instead of lines)
  • from newly created lines, delete the overlapping ones, creating a single outline
  • select the outline made from lines
  • create from selection -> create zone from selection
  • delete the previous zones and the line outline

Steps to reproduce

  1. Zones were created from selected shapes (create from selection -> create zone from selection) from imported graphics (dxf)
  2. Some of those zones can't be merged due to insufficient overlap error mentioned. Other zones made the same way can be merged. For example, I had 12V and GND zones. All of the 12V zones and none of the GND zones could be merged.

KiCad Version

Application: KiCad x64 on x64

Version: 9.0.4, release build

Libraries:
	wxWidgets 3.2.8
	FreeType 2.13.3
	HarfBuzz 10.2.0
	FontConfig 2.15.0
	libcurl/8.13.0-DEV Schannel zlib/1.3.1

Platform: Windows 11 (build 22631), 64-bit edition, 64 bit, Little endian, wxMSW
OpenGL: NVIDIA Corporation, NVIDIA GeForce RTX 3090/PCIe/SSE2, 4.6.0 NVIDIA 576.28

Build Info:
	Date: Aug 18 2025 00:41:45
	wxWidgets: 3.2.8 (wchar_t,wx containers)
	Boost: 1.88.0
	OCC: 7.9.1
	Curl: 8.13.0-DEV
	ngspice: 44
	Compiler: Visual C++ 1942 without C++ ABI
	KICAD_IPC_API=ON

Locale: 
	Lang: en_SI
	Enc: UTF-8
	Num: 1.234,5
	Encoded кΩ丈: D0BACEA9E4B888 (sys), D0BACEA9E4B888 (utf8)