There is a bug on course "enrol users" page. If you have long list of names and start to drag the vertical scroll bar down with a mouse the menu containing possible participants dissappear.
If you move the menu down with mouse scrollwheel or arrow keys menu doesn't dissappear.
Please fix it so that you can drag the vertical scroll bar with mouse.
Tested with Clean and Boost themes & Moodle 3.5.3
See the video: scroll1.mp4