1127208 - Background image jumps position when changing between active and inactive layer

Status: NEW

(Core :: Layout, defect)

Description

[Cynthia Tang [:cynthiatang]]

### ENV
Device: Flatfish (tv)
Gaia: 5dd0226e5d10433cd8f460a1019b1bfe3d886a3c

### STR
1. Pree arrow key to move the focus on any app on the Home screen 

### Actual Result
1. The name of the app juggles

### Expected Result
1. The name of the app should not juggle

Comment 1

[Sung Chiu[:sungchiu]]

Attachment: Simple example for reproducing the bug

1. When we press tab and focus the button with camera background, the button will move right-bottom a little when the scale transform is finished.
2. I found two CSS attributes that make this happen. We can either delete 'background-position: center center;' or change 'background-size: 100% auto;' to 'background-size: 100% 100%;'.

Comment 2

[Jerry Shih[:jerry] (UTC+8) (inactive)]

Ethan, please check the attachment 8561188 [details] with browser.

Comment 3

[Benoit Girard (:BenWa)]

Attachment: Bug_1127208_example 2.zip

Here's a different version of the test case that reproduces on my machine. If you watch the center of the camera icon you can see it jump at the end of the grow animation after clicking on the icon.

Comment 4

[Ethan Lin[:ethlin]]

When doing the animation, content side will normalize the scale to power of 2 (for reducing redraw and keeping quality) [1] and generate a scaled image, Image1. Then the compositor will use the Image1 to do the scale to the actual size. After finishing the animation, content side will use the real scale to generate another scaled image, Image2. The jump problem is because the centers of Image1 and Image2 are slightly different. I think this may be the content side layer snapping problem, but I'm not sure how to fix the problem. 
BTW, I try the example on chrome and it also has the jumps. 

[1]https://dxr.mozilla.org/mozilla-central/source/layout/base/FrameLayerBuilder.cpp#3934

Comment 5

[Ethan Lin[:ethlin]]

Attachment: static image example

This example has two static circles. One is scale to 1.4 and another is scale to 2.0. If we take the purple circle edge as the center, there is one pixel offset. That is why we see the jump in the end of animation.

Comment 6

[Mike Lien[:mlien]]

For TV platform, all changing between focused and de-focused

Comment 7

[Mike Lien[:mlien]]

For TV platform, all changing between focused and de-focused will be affected and the string shake obviously. This is a much worse user experience, QA suggests raise this issue's priority to fix it.

Comment 8

[BugBot [:suhaib / :marco/ :calixte]]

The bug assignee didn't login in Bugzilla in the last 7 months, so the assignee is being reset.