1880794 - Tab preview from dragging appears to use software rendering and blocks both content-process and parent process

Status: NEW

(Core :: Graphics, defect, P2)

Description

[Mayank Bansal]

1. Open google.com in tabB

2. Open the attached testcase in tabA. It will open instantaneously. (you will notice that it uses hw-wr which is expected)

3. Drag the tab from the tab bar to create a new window

AR: The tab takes a long time to draw. It also appears to use software rendering.

4. Now reattach the new window back to the original window.

AR: the tab takes a long time to draw

Profile: https://share.firefox.dev/49C1NTa , https://share.firefox.dev/3ON1exQ

Profile suggests that this drawing is using sw-wr and happens on both the content-process and parent-process.

Maybe improve this, or give-up generating tab-preview for "complex" pages, or if it takes more than 200ms or something.

Can repro on a build from Jan2022, so not a new regression. This behaviour may be by design so feel free to mark INVALID.

Comment 1

[Mayank Bansal]

Attachment: about:support

Comment 2

[Mayank Bansal]

Attachment: Testcase.html

Comment 3

[Mayank Bansal]

cc'ing some folks who may have an opinion here.

Comment 4

[BugBot [:suhaib / :marco/ :calixte]]

The severity field is not set for this bug.
:bhood, could you have a look please?

For more information, please visit BugBot documentation.

Comment 5

[Julien Wajsberg [:julienw]]

It would be good to check if this still happens (I can't do that because I believe I'm on SW WR always :D). Mayank, can you please confirm? Thanks

Comment 6

[Mayank Bansal]

I can still repro this : https://share.firefox.dev/4cQQPdC

Comment 7

[Julien Wajsberg [:julienw]]

Thanks!

The Performance Impact Calculator has determined this bug's performance impact to be low. If you'd like to request re-triage, you can reset the Performance Impact flag to "?" or needinfo the triage sheriff.

Platforms: Windows
Impact on site: Causes noticeable jank
Configuration: Specific but common
Websites affected: Rare
[x] Able to reproduce locally

Comment 8

[Nicolas Silva [:nical]]

I'm not familiar with how tab previews and drag snapshot are implemented, but from the look of these profiles they don't seem to involve WebRender. It's using the main thread panting code to call into skia directly.

Painting blurs is expensive on the CPU (that's what most of the skia time appears to be in), but I'm surprised that we'd spend so long (1.2s) when rendering a rather small thumbnail.

Comment 9

[Mayank Bansal]

Attachment: Testcase2_Torus.html

This doesnt have blurs, but still takes a lot of time : https://share.firefox.dev/4e19Paq

Comment 10

[Nicolas Silva [:nical]]

Emilio pointed me to this code: https://searchfox.org/mozilla-central/rev/82ce14a9dc278584b868b2b9c252ba0871a7782b/browser/components/tabbrowser/content/tabs.js#594
and https://searchfox.org/mozilla-central/rev/82ce14a9dc278584b868b2b9c252ba0871a7782b/toolkit/components/thumbnails/PageThumbs.sys.mjs#386-393

Comment 11

[Nicolas Silva [:nical]]

(In reply to Mayank Bansal from comment #9)

Created attachment 9422566 [details]
Testcase2_Torus.html

This doesnt have blurs, but still takes a lot of time : https://share.firefox.dev/4e19Paq

Yeah, 3d transforms is also going to be a bad time for skia (software).

Comment 12

[Mayank Bansal]

bug 1873139 is a related bug for the torus testcase.
bug 1719164 has a similar bug where the drag preview is huge and causes janks.