<http://trac.webkit.org/projects/webkit/changeset/53420> introduced a repaint regression. On this page, if you slowly scroll the bottom of the comments field into view, the bottom does not paint.
This is probably because the repaint logic considers the first and last line boxes, and the next-to-last line box has visible overflow that is greater than the last’s, which is just a TrailingFloatsRootInlineBox.
Created attachment 46846 [details]
Copy bottom overflow from the last root box to the trailing floats root box
Includes a repaint test. No change to existing repaint tests’ results.