We already get passed in the content height, so subtracting out borders/padding from that value because of box-sizing results in an incorrect value.
This breaks messenger.com on Facebook.
Created attachment 276958 [details]
This is the cause of:
<rdar://problem/23204668> REGRESSION (r189567): The top of Facebook's messenger.com looks visually broken in some window sizes
Created attachment 277063 [details]
Fixed in r199877.
Re-opened since this is blocked by bug 156918
*** Bug 156917 has been marked as a duplicate of this bug. ***
Created attachment 277081 [details]
I uploaded the wrong patch. The real fix was more low-level in order to apply everywhere.
Landed in r199895.