Many buttons in about:preferences became two-liners after landing bug 1822131. This is clearly visible in the latest Slovak Nightly build 20230315092641 (screenshot attached), there are even few buttons across the preferences UI in English build where this is visible (once you set intl.l10n.pseudo to accented you'll get even more of them).

Set release status flags based on info from the regressing bug 1822131

:emilio, since you are the author of the regressor, bug 1822131, could you take a look? Also, could you set the severity field?

For more information, please visit auto_nag documentation.

In a setup with:

<hbox>
<something flex="1"/>
<something-else/>
</hbox>

Before bug 1822131 <something flex="1"> ended up with flex-basis: auto,
but was the only thing able to shrink, so <something-else> stayed the
same size.

After that bug however <something-else> is able to shrink too, so both
elements shrink. This wouldn't happen if flex="1" actually worked like
flex: 1 does.

However flex: 1 causes stuff like explicit main sizes to be
(effectively) ignored, so we need to fix up a few cases where now we'd
start flexing too much. For that, add a debug assert to
nsFlexContainerFrame to catch the would-be behavior changes here.

For the most part they're actually no-op since they're setting tiny
sizes, but preferences and devtools needed a couple real fixes.

The profile selection spacer is useless (zero-size).

Hopefully the last xul.css change I need to do :')

Depends on D172704

FYI, comment 6 has a patch that allows to detect cases where explicit sizes used to apply and won't after this. Fix is either removing that size if it's zero or so, using flex-basis: auto (e.g. change flex="1" to style="flex: 1 auto" if you do need the size to take effect).

Hopefully the last all-xul-is-affected change I need to do. Makes the setup much clearer tho (as flex="0" and flex="1" are symmetric now).

https://hg.mozilla.org/mozilla-central/rev/b50655fcce3f