1160723 - Automatic layerization still causes unnecessary rendering

Status: RESOLVED WORKSFORME

(Core :: Graphics: Layers, defect)

Description

[Brian Birtles (:birtles)]

In the following codepen, the profiler/paint flashing shows us doing a lot of rendering even though we're clearly layerizing the moving circles:

  http://codepen.io/thebabydino/pen/JddBPE

If I add will-change to the styles as follows:

  circle {
    will-change: transform;
  }

we don't do any rendering at all. I would have expected our automatic layerization to behave much the same as providing explicit layerization hints.

(Testing on Aurora here so this might be fixed on trunk.)

Comment 1

[Matt Woodrow (:mattwoodrow)]

I only see us flashing the smaller circles.

Having a look into the debugger, we're failing the IsItemToSmallForActiveLayer check in nsDisplayTransform::GetLayerState (example rect(339, 1485, 14, 15)).

These circles aren't getting active layers, so the repainting is expected.

The will-change path in GetLayerState doesn't do this check, so we actually get active layers for all circles with that change.

Comment 2

[Brian Birtles (:birtles)]

Because I was looking at this on a smaller screen everything was in the "too small" category.

It this is the intended behavior then maybe this is invalid. It seems a little surprising that will-change has this effect. I originally filed the bug because of a report that perf was slow.[1] It seems will-change helped in this case.[2]

[1] https://twitter.com/anatudor/status/594400551359684608
[2] https://twitter.com/anatudor/status/594500949412052993

Comment 3

[Gregory Pappas [:gregp]]

This test case performs well without will-change now.