Gerber generation, inconsistent solder mask expansion around SMD rounded rectangle corners

Description

In the Gerber files generated from both KiCAD 9.0.9 and 10.0.2 there is an error in the way that solder mask expansions around rounded rectangle corners are generated. Its a small error but represents 20% expansion from 0.1mm to 0.12mm

The behavior has been observed in both KiCAD 9 and 10, the error was confirmed using the built in Gerber viewer and a 3rd party Gerber viewer.

Images: This image shows the PCB source file, solder mask expansion is the expected 0.1mm on both the flat sides and around the corner. PCB_Source_File

This image shows the settings to generate the Gerber files Plot_Settings

This image shows the inconsistent solder mask expansion in the solder mask. 0.1mm on the flat sides, 0.12mm at the corner. Generated_Gerber_FIle

This image shows the same distortion around the corner using a 3rd party Gerber viewer (pcbway.com) 3rd_Party_Gerber_Viewer

Steps to reproduce

  1. create PCB that include components with rounded rectangle SMD pads. Test expansion set at 0.1mm
  2. Generate Gerber files (Fabrication Outputs > Gerbers (.gbr)). Screenshot of settings included.
  3. Open KiCAD Gerber viewer and inspect the corners of the solder mask expansion.

KiCad Version

Application: KiCad x64 on x64

Version: 10.0.2, release build

Libraries:
	wxWidgets 3.3.2 
	FreeType 2.13.3
	HarfBuzz 12.3.0
	FontConfig 2.17.1
	libcurl/8.18.0 Schannel zlib/1.3.1

Platform: Windows 11 (build 26100), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: May  8 2026 00:53:13
	wxWidgets: 3.3.2 (wchar_t,STL containers)
	Boost: 1.90.0
	OCC: 7.9.2
	Curl: 8.18.0
	ngspice: 46
	Compiler: Visual C++ 1944 without C++ ABI
	KICAD_IPC_API=ON
	KICAD_USE_PCH=OFF

Locale: 
	Lang: en_CA
	Enc: UTF-8
	Num: 1,234.5
	Encoded кΩ丈: D0BACEA9E4B888 (sys), D0BACEA9E4B888 (utf8)