With WebRender the displayport is updated every 128px which causes very frequent scene builds.

Backed out for mochitest failures on test_group_zoom.html

Backout link: https://hg.mozilla.org/integration/autoland/rev/27e254e8a57504c284959b209eac538a35871b3c
Log link: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=301007901&repo=autoland&lineNumber=11669

https://hg.mozilla.org/mozilla-central/rev/a2e7394fa811

tscrollx is very sensible to the size of the displayport. I am trying a few approaches:

• (A) reduce the displayport for 3.5 screenfulls to 2.5 screenfulls of content
• (B) align the position and size separately (keep aligning the size to 128) to avoid enlarging the displayport

(A) alone isn't enough because large alignment still round up to large display ports. There is also more risk of checkerboarding. On the other hand it considerably reduces the CPU time spent on displaylist building scene building and frame building.
(B) in addition to (A) results in nice tp5o_scroll improvements while avoiding the tscrollx regression https://treeherder.mozilla.org/perf.html#/compare?originalProject=try&originalRevision=d2e4ea0d39f7edf3bb30f39649d8110f0ef7ce49&newProject=try&newRevision=f3a554158ca6510e0752361c50b75d5ff1db88bc&framework=1

The tp5o_scroll improvements come from the smaller displayport, this is the result of smaller displayport without the alignment changes: https://treeherder.mozilla.org/perf.html#/compare?originalProject=try&originalRevision=d2e4ea0d39f7edf3bb30f39649d8110f0ef7ce49&newProject=try&newRevision=af536c4a53122ca183d1d738a07aff26021e8bfe&framework=1

The extra checkerboarding form (A) isn't noticeable on pages like youtube's front page, but it is very noticeable on pages where the content process's main thread is very busy with layout, style or js like https://github.com/n64decomp/ido/blob/master/src/uopt/uoptemit.c
It would be great to be able to mitigate that by dynamically growing the display port when we detect that we are checkerboarding.

Waiting for results of doing (B) without (A): https://treeherder.mozilla.org/perf.html#/compare?originalProject=try&originalRevision=d2e4ea0d39f7edf3bb30f39649d8110f0ef7ce49&newProject=try&newRevision=74e5b18e7cdd26122bb1a48d190e292c2bbfc771&framework=1

All of these are with an alignment of 1024 for the displayport (position or position+size) instead of 128.

This allows moving the displayport in much larger increments (1024) without increasing the displayport size which regresses tscrollx.

Backed out changeset ab8da9fe39fe (Bug 1635472) for causing reftest failures in dynamic-toolbar-sticky-4b.html CLOSED TREE

Push with failures: https://treeherder.mozilla.org/#/jobs?repo=autoland&group_state=expanded&selectedTaskRun=SsmiJz1NSgG1qa-8RX15SA.0&resultStatus=testfailed%2Cbusted%2Cexception&revision=ab8da9fe39fe8e086a642bc967695be0bed3c6d3

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=306080329&repo=autoland&lineNumber=12237

Backout: https://hg.mozilla.org/integration/autoland/rev/cfa67d766e0b617742dd6d6fab712125ff3ee795

Other failures: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=306083987&repo=autoland&lineNumber=19330

https://hg.mozilla.org/mozilla-central/rev/7d080876c7e2