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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