Formatting toolbar buttons not visible in IE 11

[afercia]

Seems to me this is pretty new, as I haven't noticed before today. Here's how a block formatting toolbar looks like in IE 11. Notice there's a selection in the block text, so all the toolbar buttons should appear. Instead, just the "Change block type" is visible:

Actually, seems the buttons are there but seems they're "overlaid" so just one of them is visible. This is very clear when tabbing with the Tab key through the buttons: all the tooltips appear in sequence:

So seems to me it's probably a CSS issue. Any relevant, recent change to the CSS?

[brandonpayton]

Based on a search with git bisect, the breaking commit appears to be 2b6d166. I haven't had a chance to look further yet.

[brandonpayton]

This is due to position: sticky being unsupported on IE11.