User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:67.0) Gecko/20100101 Firefox/67.0

Steps to reproduce:

In an Android GeckoView browser (I used reference-browser.apk)
Turn on WebRender
Ensure picture caching is on
Set gfx.webrender.debug.picture-caching TRUE

scroll a page e.g. mozilla.org

On Android the tiles stay stationary and the content moves up through the tiles. The tiles all remain red, presumably no caching is in effect.

On Desktop, the tiles move up the page with the scrolling and so most of the tiles remain green / cached. Only the bottom row go red.

I think the Desktop case is correct, the tiles should move to avoid unnecessary GPU work.

I suspect this is the WR logic to detect the main scroll root failing on mobile.

I haven't tested picture caching on mobile yet, but I wouldn't be surprised if the display list layout coming from Gecko is different enough in structure that this logic is failing.

This is the logic that I suspect is getting the wrong scroll root - https://searchfox.org/mozilla-central/source/gfx/wr/webrender/src/display_list_flattener.rs#279

This is an awesome bug report - thank you Mark!

Please keep testing WR on Android and let us know of other issues you find.

(In reply to Glenn Watson [:gw] from comment #2)

This is the logic that I suspect is getting the wrong scroll root - https://searchfox.org/mozilla-central/source/gfx/wr/webrender/src/display_list_flattener.rs#279

Permalink: https://searchfox.org/mozilla-central/rev/152993fa346c8fd9296e4cd6622234a664f53341/gfx/wr/webrender/src/display_list_flattener.rs#279

This will be taken care of by bug 1536357.