Comment 18 for bug 125970

Actually, now that I'm looking at this, there seem to be as many as three bugs here:

(1) Like in the bug's name, any pages with fixed elements scroll noticeably
slower than those without such elements. The second patch illustrates only this
bug. Additionally, on Windows2k/XP, I can duplicate this behavior on a page
without fixed elements (such as either attachment in 'fast' mode) by having an
'always on top' window overlapping the browser window. Can this be checked in linux?

(2) In the case that such a fixed element is displayed partially offscreen, two
things happen: (i) scrolling slows drastically, and (ii) the element can
overdraw the scrollbar.

(3) This is the bug I was initially encountering/reporting, and can no longer
reproduce. While running the program jEdit 4.01, mozilla/phoenix were scrolling
at speeds of around 5-6 px/second on pages with fixed elements. If I encounter
this again, I'll make a new, more specifically targeted (and better tested!) bug.

As it stands, I think it'd be best to focus on the first of these here, perhaps
make a new bug for the second if it's unrelated, and drop the third entirely.