At least on my Windows 11 version, apps that use non-overlay scrollbars get a thicker thumb, see screenshot of the "native" scrollbars drawn by task manager (they also are drawn like this in nsNativeTheme).

We also still paint Windows 10 scrollbars on windows 11 with dark theme and "native" scrollbars, which we should probably fix.

Right arrow was being painted pointing to the wrong direction.

Having the points being centered and displace them is easier to reason
about, IMO.

Add a pref to force the small scrollbar even in
non-overlay-scrollbars mode (which is the current behavior).

Depends on D134679

Very trivial changes that make overlay scrollbars work on Windows.

Depends on D134680

Right arrow was being painted pointing to the wrong direction.

Having the points being centered and displace them is easier to reason
about, IMO.

Add a pref to force the small scrollbar even in
non-overlay-scrollbars mode (which is the current behavior).

Depends on D134682

Very trivial changes that make overlay scrollbars work on Windows.

Depends on D134683

Much like GTK. This ensures consistent scrollbar styles across Firefox.

Depends on D134684

Depends on D134685

This is useful so it doesn't look ugly with custom sizes (which might be useful
for accessibility features), but it will also be necessary for scrollbar-width:
thin.

Depends on D134689

This also makes two tweaks:

• Consistently applies hover / active feedback independent of scrollbar-color.
• Apply scrollbar-color to scrollbar buttons too.

Depends on D134690

https://hg.mozilla.org/mozilla-central/rev/0e01214655e0
https://hg.mozilla.org/mozilla-central/rev/6d3bc5473a1e
https://hg.mozilla.org/mozilla-central/rev/3f5065553270
https://hg.mozilla.org/mozilla-central/rev/56a5f901ac95
https://hg.mozilla.org/mozilla-central/rev/6c2ec190cd1f
https://hg.mozilla.org/mozilla-central/rev/4e9ae7efa612
https://hg.mozilla.org/mozilla-central/rev/051c8cc7021c