Just updated to 2018 from 2016 and I'm having a couple of issues with the UI in 3ds max. So basically i can't select drop down windows (such as export selected which is a drop down from export in the file tab) and I can't move my custom toolbar buttons from where i have put them in the toolbar (whereas in 2016 this worked fine)
Edit: SOLVED, the drop down menus speed to disappear was set extremely low. See this post for a solution