Created attachment 317249 [details]
This test shows that the top/left scroll position can be 0, due to the overflow from the native widget theme. This is being applied on Mac to buttons/textboxes/etc, so this bug is easier to reproduce on Mac. (Windows only has overflow for dropmarkers, and Linux only for tabs) It involves several characteristics:
- nsNativeTheme:GetWidgetOverflow returns a size
- The scroll position starts at 0,0 but it can be moved lower to -4,-4 using the scrollBy method.
- The 0 margin on the textbox in the testcase seems to be needed as well
This looks to be fixed.