The modifier-only-shortcuts break as soon as KWin grabs the keyboard
(e.g. alt+tab, present windows, etc.). The investigation shows that in
that case KWin does not get any raw key events any more and thus gets
confused about the state of the hold modifiers. E.g. alt+tab has the
alt key pressed, but we miss the release as the keyboard is grabbed.
This change addresses the problem by installing an additional event
filter for key press and release event which only filters for key events
on the root window. That way we can be sure that it only operates when
KWin grabbed the keyboard on the root window.
Note: the problem only exists when grabbing on the root window. If the
grab is on another window (e.g. moving a window) we still do get all
events.
The problem also seems to not happen if another application grabbed
keys on the root window. E.g. for key combinations grabbed by
kglobalaccel the correct sequence of key press/release as raw events
are reported. Also while the screen is locked the evemts are reported
and kscreenlocker grabs the keyboard on the root window.