LMarbles from rapps, pressing the 'f' key to switch into fullscreen leaves the taskbar visible

0.4.12-RC-44-gca0e00e gcc dbg lin is affected
VBox 4.3.12 + VBEMP + desktop resolution 800x600 32bpp

Reproduction steps

• Run LMarbles 1.0.6 from rapps.
• Navigate through the menu until you are 'ingame'
• Press the 'f' key on your keyboard to switch into fullscreen

Expected result
The app switches to fullscreen and the taskbar is hidden

Observed result
The app switches to fullscreen, but leaves the taskbar visible erroneously and logs

(win32ss/gdi/eng/pdevobj.c:522) PDEVOBJ_bSwitchMode, ppdev = E163E008, pSurface = E1714EF8

(win32ss/gdi/eng/pdevobj.c:585) leave, ppdev = E163E008, pSurface = E1E86D20

(win32ss/gdi/eng/mouse.c:419) Screen surface and cursor color bitmap format don't match!.

(win32ss/gdi/eng/mouse.c:419) Screen surface and cursor color bitmap format don't match!.

(win32ss/user/ntuser/display.c:868) err: BitCount New 16 Orig 32 ChkNew 16

(win32ss/user/ntuser/windc.c:588) err: Should not be called!!

I assume this is a bug in our explorer, and I would assume explorer_old and explorer_2k3sp2 run on reactOS would not show this issue, but I have not double-checked yet.

The bug is old, I noticed it for the first time during ros 0.4.3rls tests and for every release since then (always using explorer_new then).