The non-native theme does two things which, combined, cause this test to
fail.
The first one is that it has slightly bigger checkboxes than other
themes (14px vs. 13px).
The second one is that it has a 2px widget-imposed border, like Mac:
https://searchfox.org/mozilla-central/rev/0dfbe5a699cc6c73cf8c14d1aa10ba10ef3ec8fa/widget/nsNativeBasicTheme.cpp#1367-1369
Which causes its baseline to go down by that amount. This was done
intentionally in bug 1675389, though I guess it could be reconsidered.
These two things combined make the checkbox grow the line slightly in
this test-case, causing the elements to move 1px apart.
The test is intended to check that the baseline calculation of a
checkbox/radio is correct, which it is, so prevent that undesired side
effect by resetting the margin to zero.
Differential Revision: https://phabricator.services.mozilla.com/D103324
Emilio Cobos Álvarez
Jonathan Kew