400208 - nsViewManager mScrollCnt "leaks", causing flicker

Status: RESOLVED FIXED

(Core :: Web Painting, defect)

Description

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Attachment: obvious patch

nsViewManager::UpdateViewAfterScroll takes a couple of early-exit paths that don't decrement mScrollCnt. This is really bad because it means mScrollCnt will be permanently > 0. And that means we will never flush reflows/restyles when painting, which can lead to flicker.

Requesting blocking because this is hurting a new version of a major app by a certain Large Internet Company. The bug is very hard to diagnose or work around, and the patch I will attach is very safe.

Comment 1

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Attachment: test

this test shows the bug. we don't have an easy way to reftest this, though, because drawWindow flushes...

changing the width of the "i" DIV to something nonzero makes the bug go away --- when the width is zero, we're exiting early from UpdateViewAfterScroll because the rect is empty.

Comment 2

[Boris Zbarsky [:bzbarsky]]

Comment on attachment 285277 [details] [diff] [review]
obvious patch

Indeed.

Comment 3

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Comment on attachment 285277 [details] [diff] [review]
obvious patch

Simple safe fix for a bug is causing weird, hard-to-diagnose problems in web applications by Large Internet Companies

Comment 4

[Mike Beltzner [:beltzner, not reading bugmail]]

Comment on attachment 285277 [details] [diff] [review]
obvious patch

a=endgame drivers for M9 landing

Comment 5

[Robert O'Callahan (:roc) (email my personal email if necessary)]

checked in.