a13711ff7b
This was caught by Markus. When you hover over an overlay scrollbar on macOS, you'd hit this the PaintScrollbar() code-path (or rather, you won't bail out from the if (overlay && !hovered)), but macOS draws the track in PaintScrollbarTrack(). Android doesn't hit this because android scrollbars can't ever be hovered (they have pointer-events: none). This is a better fix than D222172, which didn't paint the background for the non-overlay scrollbars... This doesn't change behavior since scrollbar and the track have the same size (as cocoa scrollbars can never have buttons). Removing nsSliderFrame is a refactor for another day if ever. Differential Revision: https://phabricator.services.mozilla.com/D222194
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