Created attachment 367470 [details]
Gif of what I am seeing on my machine
Please take a look at the following page: https://reprod-safari-bug-ekufemrdfl.now.sh
Every second, a class is toggled that is intended to toggle the background-color and color properties of the #site-mast element.
In the current version of Safari, the background-color flickers. Removing the background-color CSS transition seems to fix this.
Please let me know if you need any more info, or if I can provide any more details.
Looks like we're failing to mark the layer as non-opaque when the background-color changes from something opaque to something non-opaque.
Fixed by 197654.
*** This bug has been marked as a duplicate of bug 197654 ***