1666692 - Enable partial-prerender-translate-12.html on WebRender

Status: NEW

(Core :: Layout, task)

Description

[Hiroyuki Ikezoe (:hiro)]

partial-prerender-translate-12.html is going to be landed in bug 1659227, but it fails on WebRender. Because when we cause a jank we use the previous transform animation value, but in the test case it's the first composition so that there is no previous animation value at the moment. In the case of non WebRender we use the static transform value on the target layer if there is no previous animation value.

Initially I thought we can use baseStyle for the WebRender case, it works for the particular test case, but it won't work if a paint happens in the middle of the animation. So we need to store the animation transform value at the moment when we build the display item and use it as a fallback value.

Though I am blocking bug 1656473, hopefully it's not a widely used case.