In some apps and browsers, there’s an option to enable overlay scrollbar (or is enabled by default). By saying overlay scrollbar, I mean a transparent scrollbar which hides when the mouse no longer covers it. It doesn’t move or change website content in any way like default one does in Firefox or Thunderbird.
GIF 1 contains default scrollbar in Firefox 111.0.1 (64-bit) (no option to enable the overlay scrollbar).
GIF 2 contains overlay scrollbar in Chrome 111.0.5563.11 (Official Build) (64-bit) after enabling the “Overlay Scrollbars” flag in chrome://flags.
GIF 3 contains overlay scrollbar in Spotify for Windows 18.104.22.1687.g2b3ce637 (scrollbar is enabled by default and not changeable).
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This seems to vary depending on your OS. Mac OS users have had overlay scrollbars in Firefox for ages, and it recently became supported on Windows 11 and Linux. (On Windows 11, the system setting "Automatically hide scroll bars in Windows" needs to be enabled.)
You can check whether your Firefox on Windows 11 or Linux is set to use overlay scroll bars using this method:
(A) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
More info on about:config: Configuration Editor for Firefox. Please keep in mind that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.
(B) In the search box in the page, type or paste overlay-scrollbars and pause while the list is filtered
(C) If any of the preferences is bolded, indicating that it has been customized, double-click it to toggle its value, or use the Reset button (left arching arrow) at the far right end of the row to restore the default value.
I can't tell whether overlay scrollbars are supposed to work on Windows 10. I don't use them myself.