Regression of r75407, LGeneral from rapps leaves the taskbar visible when selecting fullscreen from context menu

VBox4.3.12 + VBEMP

Reproduction steps
-set desktop resolution to 800x600 32bit
-install lgeneral-1.1-win32.exe
-copy contents of SDL-1.2.15-win32.zip & SDL_mixer-1.2.12-win32.zip into game dir
-run the game, it will start in windowed mode
-click right mouse button to open the context menu
-select options > video mode > full screen/windowed mode

Expected result
game enters fullscreen 640x480 and hides the taskbar
r75406_ok.png
r75406_ok.log

Observed result
game enters fullscreen 640x480 but leaves the taskbar visible
r75407_affected.png
r75407_affected.log

A regression, guilty revision is SVN r75407 == git 0.4.6-dev-g09ab5ea7ed2ab0e709ed2e8f8f9f6c3711b2ac7f

Side-note
Obviously related to CORE-15477, but please do not resolve as duplicate yet, because that game currently has a speciality (compared to the other 2tickets) when executed on recent revisions:
The taskbar can be overpainted, aka wiggled away, which was not possible back in
r75407_affected_butTaskbarCanNotBeWiggledAwayYet.webm