Maybe you are having trouble with Wayland on Linux or with whatever is your window manager because both of what we answered work on a normal functioning system. Qt coordinates this with the window manager.
Also, you can edit Glaxnimate’s configuration data. Use Help > About > Paths and click the icon for Settings. It opens in a text editor for me. You should close Glaxnimate before saving the changes in the text editor. Otherwise, Glaxnimate will likely overwrite the settings on close. Remove configuration key window_state to reset Glaxnimate’s layout.