Comment 12 for bug 1131385

If all of the windows shown on desktop (active windows) are set as 'always on top',
GetTopMostValidWindowInViewport() will be "Desktop" but not any of the active windows.

This could explain the phenomenon noted in the comment #9:
'if there are two windows set as "Always onTop", this bug could be reproduced.'

It looks like the flag 'Always on Top' make active windows 'could not be seen' or 'disapear from Desktop'
If we could make GetTopMostValidWindowInViewport() return the expected value,
this bug could be fixed.