473813 - Insane visual lag and high CPU usage when smooth scrolling with position:fixed elements

Status: RESOLVED DUPLICATE of bug 372039

(Firefox :: General, defect)

Description

[Robert]

User-Agent:       Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.5) Gecko/2008120122 Firefox/3.0.5
Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.5) Gecko/2008120122 Firefox/3.0.5

When smooth scrolling in a page that has a large element that has the CSS position:fixed property, the CPU usage becomes insanely high, almost always 100% for the core in use. Usually the visual lag is visible as well. I can't say this next thing with certainty, but I believe that if a page has many elements, such as a big table, the visual lag becomes worse.

I've supplied a slashdot article as an example, because the comments bar becomes fixed when you scroll down non-members and members who didn't change that setting.

Reproducible: Always

Steps to Reproduce:
1. Visit a site with a large element that has the CSS position:fixed property.
2. Make sure smooth scrolling is enabled
3. Watch the CPU usage and/or visual lag when scrolling (quickly).
Actual Results:  
Visual lag

Expected Results:  
Scrolling as if everything was dandy.