Top-left corner of header bar should only be rounded if it has the "titlebar" style class
With GTK 3 commit 850a086a, look at this screenshot:
See how there is a little rounded white space in the top-left of the second header bar? That looks like a bug to me, right? It only makes sense if the header bar is a title bar. That can be detected using the "titlebar" style class.
CC @jimmac