Bug 1408701: Ensure that we continue to show dark scrollbar thumbs on macOS 10.13 when scrollbars are always displayed. r=mstange

widget/cocoa/nsNativeThemeCocoa.mm

@@ -2778,7 +2778,8 @@ nsNativeThemeCocoa::DrawWidgetBackground(gfxContext* aContext,
         (isOverlay ? @"kCUIWidgetOverlayScrollBar" : @"scrollbar"), @"widget",
         (isSmall ? @"small" : @"regular"), @"size",
         (isHorizontal ? @"kCUIOrientHorizontal" : @"kCUIOrientVertical"), @"kCUIOrientationKey",
-        (isOnTopOfDarkBackground ? @"kCUIVariantWhite" : @""), @"kCUIVariantKey",
+        (isOverlay && isOnTopOfDarkBackground ? @"kCUIVariantWhite" : @""),
+          @"kCUIVariantKey",
         [NSNumber numberWithBool:YES], @"indiconly",
         [NSNumber numberWithBool:YES], @"kCUIThumbProportionKey",
         [NSNumber numberWithBool:YES], @"is.flipped",