Soft lock with 4 right-clicks

[ccchan49]

#KSP-RO/RP-1#1928
It seems that right clicks on UI can cause a soft lock in VAB or in-flight. I first noticed this problem when I was editing a maneuver with Principia in the map view. The game is not frozen (the spacecraft is still flying) but I am not able to control anything. Esc doesn't work too, but pressing the space bar in VAB does move the camera back to the default angle. This is how I can reliably reproduce the problem:

1: Enter VAB. Pick a core so that MJ and other UIs pop up on the screen. You can move the core away from the center of the screen, but it doesn't matter.
2: Move any UI to the center of the screen (I used KCT in this case).
3: Double-right-click on the center of the screen.
4: Double-right-click on somewhere outside of that UI.
5: The cursor disappears and the game is soft-locked.

RealAntennas's planning UI won't cause this issue