SuperTux 0.5.1 from rapps - impossible to switch back into app after having switched out via win-key

ros 0.4.9-RC-35-g3eb03ef
SuperTux was recently updated to 0.5.1 in rapps.

With the new version 0.5.1 we are capable to switch into fullscreen mode when desktop resolution is 800x600 or 1024x768. Fine.
Even the taskbar is not visible in fullscreen - very nice.
(Sidenote: Please note that the older version 0.1.3 most likely still is not capable to hide the taskbar in fullscreen)

I can leave the application in fullscreen by pressing winkey.
We return to desktop and startmenu is opened then.

Expected behavior:
There is a way to bring me back into the app. E.g. via alt+tab or by pressing the apps taskbar tile

Observed behavior
I found no way back into the app and reactos logs endlessly

(../../win32ss/user/ntuser/message.c:790) err: Message WM_PAINT count 1 Internal Paint Set? FALSE

My only way out was to kill app via taskmgr.
react.log