1248317 - APZ paints wrong position when you see bottom of a long page and enabling smooth scroll

Status: RESOLVED DUPLICATE of bug 1209100

(Core :: Panning and Zooming, defect)

Description

[Masayuki Nakano [:masayuki] (he/him)(JST, +0900)]

STR:

1. Make sure smooth scroll enabled.
2. Make sure APZ enabled.
3. Load bug 1013364.
4. Scroll to bottom (about 80% from the top, e.g., scroll to "2016-01-29 20:02:27 PST") with scroll bar.
5. Scroll to bottom with mouse wheel very slowly.
6. Move mouse cursor over a link.

Expected result:

The link looks not moved from the painted position.

Actual result:

The link is repainted different position.


I see this bug a lot of time in bugzilla and twitter.com.  If smooth scroll is disabled, I cannot reproduce this. It seems that APZ has rounding trouble with very slow scrolling caused by smooth scrolling.

[Tracking Requested - why for this release]:

If APZ will be enabled in default settings starting from 46, this bug should be fixed before that because user may click wrong element.  The difference between painted position and actual element position becomes bigger if the page is bigger and scrolled to very big scrollTop position.

Comment 1

[Markus Stange [:mstange]]

This is a compositor bug (the partial present region calculation doesn't detect the transform change as relevant).

Comment 2

[Liz Henry (:lizzard) (relman/hg->git project)]

Tracked in bug 1209100.