In the testcase you will have to resize the window that the sidebar is above the red square.
After that, try to click the radio buttons. All radio buttons that are apparently above the square (in user perspective) are unclickable.
The radio buttons that are around the square are clickable.
The attachment in this post contains the reduced testcase. Just follow the instructions above to reproduce it =)
Created attachment 96220 [details]
reduced testcase. The title of the file describes how to see the buggy behaviour.
Thanks for the testcase.
This may be the same as bug 62797.