When overlay scrollbars are enabled (macOS and Android by default, other platforms if you set the `ui.useOverlayScrollbars` pref), scrollbars don't take up any layout space. That's because [`nsScrollbarFrame::GetXULMargin`](https://searchfox.org/mozilla-central/rev/a3266997560904b484fd89d6a6bcdeace4a13652/layout/xul/nsScrollbarFrame.cpp#165-199) sets a negative margin on the scrollbar that is equal to its size. However, it only does that if `theme->ThemeSupportsWidget(presContext, this, StyleAppearance::Scrollbar)`. It seems that Windows (?), Linux and the non-native theme don't return true in this call, so overlay scrollbars incorrectly take up space.
Bug 1619428 Comment 0 Edit History
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When overlay scrollbars are enabled (macOS and Android by default, other platforms if you set the `ui.useOverlayScrollbars` pref), scrollbars shouldn't take up any layout space. The way this works on the platforms it works on is that [`nsScrollbarFrame::GetXULMargin`](https://searchfox.org/mozilla-central/rev/a3266997560904b484fd89d6a6bcdeace4a13652/layout/xul/nsScrollbarFrame.cpp#165-199) sets a negative margin on the scrollbar that is equal to its size. However, it only does that if `theme->ThemeSupportsWidget(presContext, this, StyleAppearance::Scrollbar)`. It seems that Windows (?), Linux and the non-native theme don't return true in this call, so overlay scrollbars incorrectly take up space.