See attached screenshot

Regression window:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=038ebfdd141acdc7603d2378a832a572bb267686&tochange=9644ee96a51a03a8f4e50e9777fb7278f246383c

[Tracking Requested - why for this release]: We can't ship this.

There used to be a class called .numberbox-input that would apply to number input inside of the xbl bindings, and that was updated to use xul|*:root html|input[type="number"] in Bug 1437641 / https://hg.mozilla.org/mozilla-central/rev/00d33e7058c1#l31.46.

After Bug 1492582, we don't have a XUL root anymore. We could restore that class and set it in the print preview instance (and any other consumers that are inside of mox-box) then replace those XUL root selectors with the class instead: https://searchfox.org/mozilla-central/search?q=xul%7C*%3Aroot+html%7Cinput%5Btype%3D%22number%22%5D&path=.

The selectors that changed number inputs when they were used in a XUL
root document no longer make sense now that we're using an html root.
Convert the selector to a class instead. Also, add flex attribute so
the toolbar fills the whole width of the wrapping box.

My

And the right most background of toolbar is also regressed by Bug 1492582

https://hg.mozilla.org/mozilla-central/rev/6127c2544d85