In issue #1705 we added support for CSS’s scrollbar-width with basic support for
Thin styling needs platform-dependent implementation. Basic support was added in that issue but could be improved by using custom styling on the thin scrollbars on Mac OS and GTK because native styling doesn’t respond as well or as consistent to changing the width of scrollbars as on Windows where this isn’t a problem.
Relevant bug in BZ to use as reference:
cocoa seems to be https://bugzilla.mozilla.org/show_bug.cgi?id=1484565
GTK seems to be https://bugzilla.mozilla.org/show_bug.cgi?id=1464723 where the implementation is blended in with scrollbar-color stuff (which I don’t see a point in implementing because nobody supports it and it’s pointless and clashes with native theming) so that needs some detangling.
Overall, this shouldn’t be too difficult but will have to be done by people who can develop on the target platforms to tweak the styling and implementation properly with direct feedback.