Here's how this looks with overlay-style scrollbars (with the pref set to false). I'm hovering the main chat area which makes the scrollbar briefly show up there, but otherwise the scrollbars get out of the way (which is similar to the behavior that Discord's full suite of ::-webkit-scrollbar rules end up giving in Chrome).
Bug 2029246 Comment 3 Edit History
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Here's how this looks with overlay-style scrollbars (with the pref set to false). I'm hovering the main chat area which makes the scrollbar briefly show up there (and hovering the server list and member list will make scrollbars show up there too); but otherwise the scrollbars get out of the way (which is similar to the behavior that Discord's full suite of ::-webkit-scrollbar rules end up giving in Chrome).