Bug 1821920 and bug 1821871 are instances of an interesting behavior
change from bug 1820534.
The default flex-basis of old XUL was auto instead of max-content,
because of this code:
https://searchfox.org/mozilla-central/rev/af78418c4b5f2c8721d1a06486cf4cf0b33e1e8d/layout/generic/nsFlexContainerFrame.cpp#1327
So stuff that used to wrap now no longer does, in an horizontal flex
container, since xul.css prevents XUL elements from shrinking.
Per the comment, a few tests relied on this, but I believe it should
generally be safe to shrink the items. This only causes to shrink if
they have an explicit width but no min-width (including min-width:
auto).
Some tests like test_mousescroll.xhtml hit this, because they have
explicit sizes but min-width: auto ends up being 0 effectively, but I
believe we should tweak those tests instead.
Differential Revision: https://phabricator.services.mozilla.com/D172462
