Regression of r75407, It depends on the desktop color-depth bpp whether taskbar is hidden in fullscreen of Anno 1602

ros 0.4.10-RC-13-g3a1fe9d gcc dbg win, VBox 4.3.40 with VBEMP

game "Anno 1602", happens with both: the full version and the demo from
https://www.fileplanet.com/35099/30000/fileinfo/1602-A.D.-Demo
(The demo runs worse than the full version but is sufficient to reproduce this ros-bug.)

"It depends on the desktop color-depth bpp whether taskbar is hidden in fullscreen"

When I have desktop bitdepth set to 32bit colors the taskbar is not hidden when the application starts in fullscreen, like in 32bit.log and taskbarVisible.png.
Contrary when I have desktop bitdepth set to 256colors, the taskbar will always be properly hidden when starting the app in fullscreen, like in 256colors.log

2k3sp2 manages to always hide the taskbar ofc.
The bug is minor for most apps but is very annoying in Anno, because ingame the game does not allow to scroll to the bottom whenever the taskbar is not properly hidden, which makes the game unplayable.

Expected result
I consider the bug as resolved, when Anno1602 can be launched from 32bit desktop and the taskbar is not visible.

Workaround1
Set the desktop bpp to 256 before starting the game

Workaround2
Kill explorer before starting the game via cmd

Workaround3
Use explorer_old as shell instead of explorer_new

Workaround4
Use explorer.exe from W2k3sp2 as shell

Side-note
I initially hoped desktop bpp would explain our taskbar-visibility-issue in general, but unfortunately other programs do still show this issue (regardless of desktop bpp).