When the mouse pointer is positioned over an iframe's border, the scroll wheel does not scroll the frame (if it is scrollable) nor the containing page. To reproduce, open the test case and follow the instructions.
Created attachment 15420 [details] Test case
Created attachment 15421 [details] Reject scroll wheel events outside the widget's view This is the kind of change that makes me think I may be overlooking something.
One thing that I'm not overlooking is that the behavior of scrollable overflows is different: when the cursor is in the border, the scroll wheel scrolls the contents of the overflow. I consider that a bug.
Comment on attachment 15421 [details] Reject scroll wheel events outside the widget's view r=me
Landed in r24098.