Code:
MainMenuBar.ignoreFramePositionManager = true
did not help much unfortunately. The MainMenuBar would still be reset constantly.
The glithc with the MainMenuBar not being clickable was that when I call
Code:
MainMenuBar:SetUserPlaced(true)
Blizzards code then does not properly set the MainMenuBar after leaving a vehicle but it is shown, as not clickable. MainMenuBar:GetRight() returns nil then. Maybe it is still hidden. I actually can fix it with setting its points.
Code:
MainMenuBar:SetPoints(...
But only after combat ends of course, so that is not a fix.
Anyway,
the fix I came up with was to set SetUserPlaced back to false when entering a vehicle. And if in combat then wait until end of combat before moving the MainMenuBar and setting SetUserPlaced to true again.
What I missed though is that that I used the event UNIT_ENTERING_VEHICLE to set SetUserPlaced(false). But that is too early because PLAYER_REGEN_ENABLED is triggered after that despite still in combat and I had that hocked to call SetUserPlaced(true) again....