I think this is wontfix as per the above, but feel free to reopen if you disagree. This shouldn't affect performance for most users (default GTK scrollbars are 12px since long ago, and I expect most users to use windows bigger than 12px).
Bug 1690494 Comment 8 Edit History
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I think this is wontfix as per the above, but feel free to reopen if you disagree. This shouldn't affect performance for most users (default GTK scrollbars are 12px since long ago, and I expect most users to use windows different to 1024px).