Gvimdiff window opens one or two columns smaller every time it is opened

Steps to reproduce

1. From a terminal type gvimdiff A B (files do not need to exist)
2. From gvim type :echo winwidth(0)
3. Note the value
4. Close gvim
5. From a terminal type gvimdiff A B
6. From gvim type :echo winwidth(0)
7. Note the value is less than the previous value

Expected behaviour

Expect the newly opened window to be the same size as the previously closed window.

Version of MacVim

MacVim r181 (Vim 9.1.1128)

MacVim installation method

brew install macvim

Environment

macOS: 15.7.1 (24G231)
mac: MacBook Pro M4
terminal: Version 2.14 (455.1)

Logs and stack traces

Vim and MacVim configuration where issue is reproducable

No response

How was MacVim launched?

• by launching MacVim.app in macOS (by using the Dock, Spotlight, or the open command)
• by running mvim/gvim in the terminal

Issue has been tested with clean configuration

• by running mvim --clean (or gvim, supplied by MacVim distribution). You can also use the File → "New Clean Window" menu item within MacVim.
• by running vim --clean (in terminal, supplied by MacVim distribution)
• by running vim --clean (in terminal, other suppliers, e.g. /usr/bin/vim)

No source file or test is identified in the report. Reproduce the shrinking width with gvimdiff A B, compare winwidth(0) across launches, and inspect the MacVim GUI launch and window-restoration entry points; done means repeated launches preserve the previously closed window width.