OpenTTD 1.8.0 from rapps does not properly hide the taskbar when fullscreen is activated

0.4.12-RC-44-gca0e00e gcc dbg linux, VBox4.3.12 + VBEMP, no additions, no 3D accel
desktop resolution 800x600 32bpp

OpenTTD 1.8.0 from rapps

The game starts up in windowed mode.
When you enter the setup screen and select 640x480 with fullscreen, then the game switches into fullscreen, but leaves the taskbar visible erroneously.

(win32ss/gdi/eng/pdevobj.c:522) PDEVOBJ_bSwitchMode, ppdev = E24C29A0, pSurface = E24C2208

(win32ss/gdi/eng/pdevobj.c:585) leave, ppdev = E24C29A0, pSurface = E27DEDD8

(win32ss/user/ntuser/msgqueue.c:1271) err: MsqSendMessage timed out 2 Status 102

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

The bug is old, I noticed it the first time in ros 0.4.0 rls test and for every rls since then.
I guess (but did not try myself) that it would work with explorer_old and explorer from 2k3sp2.

Expected result
2K3_SP2_ok.png