Used Linux Mint 17.3 as host and Windows 8.1 as guest.
Problem: dropdown lists seem to scroll to the last item on their own and it does not happen all the time, it appears random.
Cause: not sure if it is the root cause but strangely enough it was not mouse driver or 3D acceleration settings as advised on some forums, in my case it appeared to be keyboard layouts. I had 2 languages for input (on host OS) – English and Russian. So when host switches to Russian language – the problem manifests, when host input is English everything is normal.
To fix it I now work with VM only when my host is set to English and I have also changed keyboard shortcuts for switching languages so they are different in host and guest (guest also has 2 input languages).
I have also noticed that any mouse movement results in scrolling, so you move the mouse and dropdown list scrolls – this is only when host OS input language was not English. That’s it for the problem and resolution, hopefully I will save some time on this thing next time.