DosBox fullscreen issue - stays windowed, does not longer update window contents

symptom when pressing alt+enter for fullscreen improved with r71318
It no longer crashes CORE-10736 or closes CORE-10737.

But it will still not switch to fullscreen on alt+enter.
It tries to switch something, then still is windowed, there is no content drawn then within the window anymore, but DosBox still accepts your blindly typed commands like "exit".