When I open a video the first time after opening Avidemux, and the window is maximized, the window shrinks after I select the video to open. Windows treats the window as if it was still maximized, so I have to click the restore button twice to bring it back to normal.
Here's a video showing the bug (https://imgur.com/Sxwjcba)
This bug was fixed a long time ago, please use a recent nightly (https://avidemux.org/nightly/).