Closed Bug 1594364 Opened 3 months ago Closed 3 months ago

Protonmail text no longer using subpixel anti-aliasing where it does on Chrome


(Core :: Graphics: WebRender, defect)

72 Branch
Not set



Tracking Status
firefox-esr68 --- unaffected
firefox70 --- unaffected
firefox71 --- unaffected
firefox72 --- fixed


(Reporter: ke5trel, Assigned: gw)




(Keywords: regression)


(2 files)


  1. With WebRender compositor on Ubuntu 19.10.
  2. Go to
  3. Inspect the text with a magnifying glass.

Text is using grayscale anti-aliasing instead of subpixel.

Does not happen with Basic compositor. Works on Chrome.

Regression range:

Regressed by Bug 1590284.

Flags: needinfo?(gwatson)
Closed: 3 months ago
Resolution: --- → DUPLICATE
Duplicate of bug: 1593216

This is a specific instance of the problem that we want to understand and perhaps deal with differently than bug 1593216

Resolution: DUPLICATE → ---

Confirmed - will take this and work out what the differences are.

Assignee: nobody → gwatson
Flags: needinfo?(gwatson)

Interestingly, Chrome decides not to create a compositor surface for the large(ish) fixed position images that show up during scrolling as a parallax effect. I'm not sure exactly what determines that yet - it does create a compositor surface for cases like, disabling subpixel AA in that case.

I prototyped not creating a compositor surface in WR for this case, and found that everything was being invalidated and rasterized every frame. Which led me to investigate and find the root cause - With that fix in place, when WR renders the protonmail page as a single compositor layer, the invalidation works as expected, just rasterizing the fixed position elements as you scroll past it. Once that lands, it will be much more feasible / efficient to not create compositor layers for fixed position elements in cases like this.

Instead of creating a picture caching slice for any content that
is fixed position, also check if the clip(s) for the cluster are
fixed position or anchored to the scroll root.

This prevents WR creating slices for parallax style effects. There's
not much point in doing this anyway, since those slices will invalidate
due to the spatial node of the clip changing during scrolling. This
also allows subpixel AA to be enabled in more situations.

Pushed by
Support subpx AA with picture caching in more cases. r=kvark
Closed: 3 months ago3 months ago
Resolution: --- → FIXED
Target Milestone: --- → mozilla72
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