Mouse drags/selects instead of just clicking (like if I had not released the button), only for VirtualBox program

Whenever I click anywhere in Virtualbox's main window (the window that lists your VMs), it starts dragging or selecting, like if I had not released the mouse button:

In the screenshot above, I performed a left-click (press then release) on the first VM, so it should just be selected. But instead, it is like I was trying to drag the VM somewhere.

Investigation:

• Fearing a hardware problem, I tried using an external USB mouse, but it does not change anything.
• In Settings>Pointing & Clicking, both Mouse Keys and Click Assist are OFF.
• VirtualBox is the only program where that happens. Right now I am using Firefox and GIMP to take the above screenshot, clicks work perfectly. Clementine (which I believe is another Qt app) also works fine.
• If I start a Windows VM, the problem also occurs inside that Windows environment.
• I am using Ubuntu 18.04.1 LTS with everything left by default, in particular I have not changed the shell/desktop environment.

How to investigate and fix that?

VirtualBox version: 5.2.10_Ubuntu r121806 Qt5.9.5

Solution 1:

Had the same problem recently, both in Virtualbox and Nixnote2 (also a Qt app). Only thing that helped was logging off and back on again.