It seems that in the latest canary build (68.0.3440.0) the cast popup is blank when MacViews is turned off.

At first, I suspected my patch:
https://chromium-review.googlesource.com/c/chromium/src/+/1070476

but I verified that in my local build of Chromium on Mac from when I landed the patch it worked but not in the canary. There's possibly something else that landed after that.

A bunch of compositing related Mac patches landed recently so I'm wondering if it's maybe one of them...