Dropdown menus can't scroll

The problem described in MDL-75575 seems to be a more general problem affecting all dropdown menus in Moodle.

Every time you have a dropdown that does not fit completely to the screen, you can't use the parts that are not visible because the menu does not scroll, neither horizontally nor vertically. This is especially a problem on mobile devices.

Examples for this problem: 

When you have a course that has a long shortname, the "More" menu in the course gets unusable on small screens:

When you have custom fields enabled in block_myoverview and have a lot of entries there, you can only use the visible part of the menu.

This problem turns to a horizontal scrolling problem on small screens:

As the latter one is more difficult to reproduce (see MDL-75575 for details), see the following steps to reproduce the first one:

1. Create a course with a shortname that uses the maximum available length.
2. Visit the course with a small display device (e.g. a mobile phone).
3. Click on "More"
4. You will only see one menu item in parts (the one to leave the course), the other menu entries are only blank spaces.
5. You won't be able to scroll inside the dropdown menu.

 This problem might affect all dropdown menus in Moodle.