Twitter triggers the slow scrolling path, which has a different ChromeClient end point. This means that currently when you scroll on Twitter with the mouse wheel, the adjustments are not updated.
Created attachment 93691 [details] Patch
Comment on attachment 93691 [details] Patch Bam.
Comment on attachment 93691 [details] Patch Clearing flags on attachment: 93691 Committed r86627: <http://trac.webkit.org/changeset/86627>
All reviewed patches have been landed. Closing bug.