When viewing this page, go down to where it says "IF YOU WILL BE ACCOMPANIED BY A GUEST CLICK HERE." Click it and new content appears below and the scrollbar adjusts. Click it again to hide that content and the scrollbar should adjust, but it does not. It leaves a bunch of extra space at the bottom of the page. Upon reviewing the nightlies. It last worked with r19843. r19850-r20361 is broken. I couldn't check r19849 & r19848 since those releases didn't render pages to the screen.
Created attachment 13740 [details] Reduction
Created attachment 13746 [details] Let the document shrink back Includes layout test and change log. No layout test regressions.
Comment on attachment 13746 [details] Let the document shrink back r=me
Committed revision 20380.