In my last post, I showed off some CSS fun with border-radius and overflow:hidden. Truth be told, that post started out about a bug with WebKit based browsers in relation to those attributes, but when I got down to writing the post, I couldn't reproduce the bug. Today, while trying to animate it, I managed to isolate the problem, and it turns out that I'm not the only one.
The problem in brief is that the contents of a container with overflow:hidden will still overflow its border if you have border-radius set and position set to anything other than static. Bug 50072 on WebKit describes a similar issue in more detail, and I've posted my comments at the bottom of that bug. I've created a minimal test case on JSFiddle.
The problem only shows up when you set the container's position style attribute to something other than static (which is the default).
I've tested in the latest versions of Firefox, Chrome, Safari and Opera, and Firefox is the only one that renders it correctly.
Now before you go asking me to test on the latest WebKit, rest assured that it's compiling as I write this. I'll try and submit a fix if it still exists and I can figure out what needs patching, but given that position:static works correctly, it may just be a matter of seeing what's different with those two flows.
Update 2016-10-28: This appears to be fixed on the latest versions of Chrome, Firefox, Safari and Opera: