As you can see on the attached image, Avidemux window extends under the Dock if it is not placed at Left but at Bottom.
avidemux dock.png
Does this happen with a build off the latest git master as well (probably yes)? You can find the appImage of the last available developer build – the one from 2025-05-30 – here (https://avidemux.org/nightly/appImage4Buster/).
What is the native resolution of your screen? You should reduce the font size at least for Qt applications as the available display height is clearly insufficient at current font size.
If the dpi value is higher than 100, this may let Qt think it needs to enable HiDPI scaling. You may try to launch Avidemux from the Terminal with the environmental variable QT_ENABLE_HIGHDPI_SCALING set to 0.