Win+arrow keys are handled by Windows itself - Win+Shift+Left/Right work for any window, and Win+Left/Win+Right work for windows with specific setup (which borderless GameMaker windows do not have).
If the window is maximized or in borderless fullscreen mode, I try to make sure that it covers the appropriate (workspace/full) screen of the containing monitor, but movement is up to the system.
Depending on your use case, this extension is already half-obsolete due to introduction of window_set_showborder function in GM2023.4