Desktop size not correct if 2nd screen is positioned below 1st screen

I am using a tablet and have connected a screen to it. Most configurations put the screens next to each other but I like to work with the tablet screen below the main screen.
Only problem is that the desktop size is not recognized correctly by any program. Normally a maximized window would stop at the "task bar". With a screen below no window stops at the task bar but at the bottom of the screen. So I always see only half of the downloaded files bar in Chrome for example.
In the attached image you can see Pluma on both screens. On the main screen you can see that the bottom bar in pluma is not visible because it is under the task bar.

How can I fix this? Or is it a bug in mate?