UI drop-down menu odd

[nomadscientist]

Describe the bug
UI drop-down menu can lose datasets if the drop-down has too many things in it.

To Reproduce
Steps to reproduce the behavior:

1. Go to usegalaxy.eu or usegalaxy.org.au
2. Try to run any tool with a drop-down menu where you have a lot of options in your history
3. See error
   

Expected behavior
The scroll bar can sometimes get lost if it is too high / too low

Additional context
Reported by new user at Galaxy SPOC Co-Fest, and verified by two others.

[mvdbeek]

This might be due to the sticky class of the tool form header. higher z-index on the select might work ?

[davelopez]

It should be fixed in #19309, thank you!