1797272 - Dragging a tab through the toolbar has become janky when there are large numbers of tabs and at least one pinned tab (especially on Wayland)

Status: RESOLVED FIXED

(Firefox :: Theme, defect)

Description

[Kestrel]

Attachment: Video of dragging tab on Wayland with 2000 tabs showing delayed response when there is a pinned tab

STR:

1. Start with MOZ_ENABLE_WAYLAND=1 on Ubuntu 22.04.
2. Open a session with 2000 tabs.
3. Pin at least one tab.
4. Drag a tab to different positions in the toolbar at various speeds.

The previously smooth tab dragging has become janky and gets progressively worse with increasing number of tabs. It is much more severe on Linux Wayland than XWayland/X11 and can make the entire browser unresponsive for some time.

It does not happen when there are no pinned tabs, except for when the dragged tab hits the far right end of the strip.

Performance profile flame graph shows a lot of time spent in nsFlexContainerFrame layout reflow:
https://share.firefox.dev/3N1ApUk

Moving tabs with the keyboard shortcut (Ctrl+Shift+Page Up/Down) is unaffected.

Turning off tab animations with ui.prefersReducedMotion = 1 makes no difference.

Regression window:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=4ff88d45f59c4357dcc71ddf4ad5344a38f9944d&tochange=aa325161aae5e555cab4d44b6662bbe029d26a5d

Regressed by Bug 1790616.

Comment 1

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

:emilio, since you are the author of the regressor, bug 1790616, could you take a look? Also, could you set the severity field?

For more information, please visit auto_nag documentation.

Comment 2

[Emilio Cobos Álvarez (:emilio)]

Attachment: Bug 1797272 - Fix flex container dynamic overflow updates to avoid triggering extra reflows. r=TYLin,dholbert

Move the custom overflow code to UnionChildOverflow, so that it's
computed correctly from UpdateOverflow() when a transformed descendant
changes.

This can cause extra reflows in some cases. In the case of the tab bar
it was here:

https://searchfox.org/mozilla-central/rev/12a18f7e112a4dcf88d8441d439b84144bfbe9a3/layout/generic/nsGfxScrollFrame.cpp#6916

The subsequent reflow ended up fixing up the overflow areas, so I don't
think it's a correctness issue otherwise.

It seems grid has the same issue, but it seems a bit harder to fix
because of subgrid and masonry, so I'll file a follow-up bug for that.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/d48aaf20cd4f
Fix flex container dynamic overflow updates to avoid triggering extra reflows. r=TYLin

Comment 4

[Iulian Moraru]

https://hg.mozilla.org/mozilla-central/rev/d48aaf20cd4f