Intent to Ship: Tab Warming (Windows and Linux)

Mike Conley mconley at
Tue Apr 24 19:11:47 UTC 2018

Hi folks,

Tab warming is a perceived performance optimization where we render and
upload layers for a tab when hovering that tab with the mouse, to
optimize for tab switching when the user actually clicks a mouse button.

Since bug 1423220[1], this optimization has been enabled on the Nightly
channel for almost all of the 61 Nightly cycle. Telemetry has
confirmed[2] a nice drop in the mean and median time between users
requesting a tab switch, and users being presented with the contents of
the requested tab. We've also seen a nice drop in the number of users on
our Nightly channel that see tab switch spinners[3].

Since baking on Nightly, only one major behavioural glitch has been
reported[4] (and subsequently fixed).

Bug 1453080[5] tracks a regression where warming can cause animations in
the tab strip on macOS to seem sluggish.

Considering that tab switching, tearing and re-ordering is something our
users do every day, I'm quite confident that simply existing on Nightly
has given us decent testing coverage. I therefore intend to land a patch
in bug 1456602[6] later this week (before the soft freeze begins) to let
tab warming ride the trains out to release on the 61 train for Windows
and Linux. Warming will continue to be enabled on Nightly for macOS, but
will be restricted to Nightly for now, until we can get bug 1453080 sorted.

Disabling the optimization, should the need arise, is trivial[7], but I
hope it doesn't come to that.

Do let me know if you have any concerns. Thanks!


[3]: - "Affected client %".
Sorry for the noisy graph / crowded labels. March 13th is when the
optimization was first detected.
[7]: The optimization is controlled via the
browser.tabs.remote.warmup.enabled pref.

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