Thanks for Avidemux. This tool helps me a lot.
Yesterday, I tried to use Avidemux 2.8.1 on my wife's computer, but that was impossible.
The minimum size of the application window was covering almost the full screen, and
important parts of the UI was below the screen.
She has an ordinary laptop, with Windows 11, screen size 1920*1080.
BUT, in the Windows Display settings, she had a screen zoom set to 150%, rather than the "Windows recommended" setting of 125%.
1) Unrelated to the GUI scaling problem you experience, but vastly beneficial for overall experience and stability: please use the latest 2.8.2 "nightly" build rather than the last, meanwhile very outdated 2.8.1 release version.
https://avidemux.org/nightly/vsWin64/
2) Follow the method mentioned in https://avidemux.org/smif/index.php/topic,20617.0.html