This was pre-existing but caught by some scrollbar-width tests. A test-case that reproduces on current nightly: data:text/html,<html style="overflow: scroll; scrollbar-color: red red; scrollbar-width: thin"> Toggling the `scrollbar-width` declaration in devtools doesn't change the effective scrollbar width. Ensure we correctly reflow the scrollbars in this case. Differential Revision: https://phabricator.services.mozilla.com/D128515
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