Until now WebRenderScrollData was only used with "layers" WR transactions, but we want to use it with layers-free transactions as well. As such, we need to allow collecting information from display items instead of layers. This restructures the code a little bit to allow that. This patch should not have any functional effect on the "layers" codepath, but on the "layers-free" codepath it is now actually populating some rudimentary data into the WebRenderScrollData before sending it across. This will be fleshed out in future patches. Review commit: https://reviewboard.mozilla.org/r/161518/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/161518/

The semantics of the WebRenderScrollData structure is that the per-layer structures form a tree with a single root node. When we build the data structure from the display list, we are generating (for now) a flat list. Therefore we need to synthesize a root node in order to make stuff work as intended. Review commit: https://reviewboard.mozilla.org/r/161520/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/161520/

This adds handling for nsDisplayRemote frames, so that we create a WebRenderLayerScrollData item for each nsDisplayRemote frame that we encounter. This is the equivalent of a "ref layer" in a normal layer tree, and allows the APZ side to glue together the scroll data from different processes into a full tree. Review commit: https://reviewboard.mozilla.org/r/161522/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/161522/

Review commit: https://reviewboard.mozilla.org/r/161524/diff/#index_header See other reviews: https://reviewboard.mozilla.org/r/161524/

Comment on attachment 8890401 [details] Bug 1384181 - Reorganize the WebRenderScrollData code to work with layers-free transactions. Review request updated; see interdiff: https://reviewboard.mozilla.org/r/161518/diff/1-2/

Comment on attachment 8890402 [details] Bug 1384181 - Ensure that the WebRenderScrollData has a single root layer node. Review request updated; see interdiff: https://reviewboard.mozilla.org/r/161520/diff/1-2/

Comment on attachment 8890403 [details] Bug 1384181 - Build WebRenderLayerScrollData instances for remote layer trees. Review request updated; see interdiff: https://reviewboard.mozilla.org/r/161522/diff/1-2/

Comment on attachment 8890404 [details] Bug 1384181 - Add utility dump functions for debugging. Review request updated; see interdiff: https://reviewboard.mozilla.org/r/161524/diff/1-2/