Hi Arne,
I noticed an issue with HiDPI scaling on Linux.
When the GDK_DPI_SCALE environment variable is set (in my case to 1.25), the UI is scaled up, but window size does not change accordingly, which moves part of the UI outside of the visible window area. This makes it impossible to click the start button, since the window cannot be resized:

A quick workaround would be to add unset GDK_DPI_SCALE to the start script.
This would make the gamecontroller usable, but is obviously not ideal, since the UI will not be scaled and could end up rather small.
Best,
Jens