On Virtualbox running Ubuntu I've encountered several times bug where suddenly top left quarter of the desktop becomes unresponsive to mouse clicks, what is causing this and how could I fix this?
This problem seems to be related to VBoxClient process being stuck, simply looking this from process list and ending the process seems to work as a fix.
For me killing instances ran with --draganddrop parameter restored the functionality of that area.
You can open terminal using keyboard combination of ctrl+alt+t, and type:
ps aux www | grep VBoxClient --
to look for the process ID (PID) then kill it with
kill -9 PID
pkill VBoxClient might do the trick.
After this to restore drag and drop support you can run
For reference here is xwininfo of the un-clickable area:
xwininfo: Window id: 0x1200001 "VBoxClientWndDnD" Absolute upper-left X: 0 Absolute upper-left Y: 0 Relative upper-left X: 0 Relative upper-left Y: 0 Width: 800 Height: 600 Depth: 0 Visual: 0x21 Visual Class: TrueColor Border width: 0 Class: InputOnly Colormap: 0x0 (not installed) Bit Gravity State: ForgetGravity Window Gravity State: NorthWestGravity Backing Store State: NotUseful Save Under State: no Map State: IsViewable Override Redirect State: yes Corners: +0+0 -1120+0 -1120-375 +0-375 -geometry 800x600+0+0