If you don't spend any time on Nightly in Windows 10, please feel free
to disregard this.
tl;dr: we're sometimes creating the first window differently than usual,
so be on the lookout for breakages.
On 2021-01-05, a change landed in Nightly which enabled the pre-XUL
skeleton UI [1]. This is a feature which allows us to create the first
window and populate it with a non-interactive placeholder UI before we
load xul.dll. On some systems, this can mean we can give visual
indication of Firefox launching as much as 15 seconds sooner than normal
(loading xul.dll can take a while). We're hoping this could be a big win
for users who experience very slow startups, and we also hope it will
improve the overall snappiness of startup even on fast systems.
For a number of reasons, though, getting this to work properly was not
trivial. We implement it by creating a window very early in startup [2],
and then passing its handle off to nsWindow the first time we would
otherwise create the first real browser window [3]. We then would like
to treat the window handle exactly as if we had just created it, despite
the fact that we did not. Discrepancies arise due to, for example, us
having already shown the window, meaning the native events associated
with doing so have already come through, and we can't process them to
set up our state as we normally would.
Accordingly, we do a little dance to try to get our state synced up with
how it ought to be if we had just created the window, and we do things a
little bit differently when we would normally be showing said window for
the first time (because we've actually already shown it). All of this is
complexity and things we could get wrong, and it could have consequences
on how the window behaves down the line during normal use. For example,
in the first iteration of this, we weren't receiving our initial focus
event on the browser window, which resulted in the text caret not
showing up until closing and reopening the browser [4].
We've done our best to track down and identify all such issues, but it's
certainly possible that more could manifest. So please don't hesitate to
reach out or file a bug under the meta [5].
Lastly, a big thanks to Emma Malysz who was instrumental in getting this
ready to land, to the Windows Widget folks who reviewed it, and to
everyone who filed bugs with it during early dogfooding.
Cheers,
dthayer
[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1680258#c15
[2]:
https://searchfox.org/mozilla-central/rev/ef900cd2258d4c5d968093f612f807d96e6e7c98/mozglue/misc/PreXULSkeletonUI.cpp#1933
[3]:
https://searchfox.org/mozilla-central/rev/ef900cd2258d4c5d968093f612f807d96e6e7c98/widget/windows/nsWindow.cpp#897
[4]: https://bugzilla.mozilla.org/show_bug.cgi?id=1665456
[5]: https://bugzilla.mozilla.org/show_bug.cgi?id=1665451
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