I have now managed to set up one of my test computers with a new Windows 11 installation. The issue can be provoked immediately without an additional program, such as firefox, being installed.
So it must be Start11 itself, it can't be otherwise.
Maybe Start11 works in a virtual machine, but causes this problem with Windows installed on hardware.
Have you the possibility to install it directly on a computer (ideally with an AMD-CPU) and try it out?
Another hint: If I go to “Taskbar / Configure extended taskbar” and switch the style to “Windows-10-style”, I can undock and redock the window at the top edge after an Explorer reset and the window is maximized again (without restarting). If the option is set to “Windows-11-style”, the gap remains at the bottom and the window is not maximized.
As I have already written, the issue only occurs in the middle taskbar size. Perhaps something went wrong there during programming.
This is now the case with 3 computers. Each has an AMD-CPU (Ryzen 5 2600 and newer). I can't try this on a system with an Intel-CPU because I don't have such a system. This may also be important.
Regards Holger