Fix scroll and Cancel icon hover issues with dropdown menus app-wide

Summary

Dropdowns with headers:

  • allow the drop down header to scroll when it should not scroll at all
  • show an odd blue halo when clicking the cancel icon at right

Steps to reproduce

  1. Goto Repository/Files, and dropdown at top that says master.
  2. Hover over the header, "Switch branch" and scroll with your mouse. Note the header scrolls slightly (and awkwardly)
  3. Click the x icon and note the odd blue highlight that shows on-click

Relevant logs and/or screenshots

Header scrolled up: Screen_Shot_2019-05-21_at_5.20.51_PM

Example of cancel highlight (which also illustrates scroll issue):

Global_dropwon_close_arrow_highlight_on_click

Edited by Shane Bouchard