I'm on the "Add Alias" page, and when I click the domain dropdown (I have many domains), that DIV scrolls and the dropdown is partially hidden until I scroll. It's very strange.

I fixed it by inspecting the page in Firefox dev tools, and I found this div:

<div class="tab-content">

I unchecked the overflow: auto to disable it, and that fixed it. So you'll probably just need to override .tab-content in the CSS and specify overflow: inherit.