When clicking on a drop-down list and releasing the mouse button right away, the drop-down list opens and remains opened (which is the expected behaviour).
When clicking on a drop-down list and keeping the mouse-button pressed, the drop-down list opens and remains opened. When releasing the mouse button the drop-down list closes, again.

For reproducing you can use any drop-down listbox, e.g. "Tools > Settings > General > General > Save Session on program exit"
I guess Qt causes this problem; it would be interesting to know whether other people also experience this problem and whether this also happens in Windows 10.
Windows 7 6.1.7600 (running in VMWare virtual machine)
XnView MP: 0.83