Bug 1868868 - The value of thumb position changed by reflow should be consistent to nsSliderFrame::CurrentPositionChanged, r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D195815

Author: cathiechen

layout/xul/nsSliderFrame.cpp

@@ -614,6 +614,13 @@ void nsSliderFrame::Reflow(nsPresContext* aPresContext,
     thumbPos.y = NSToCoordRound(pos * mRatio);
   }
 
+  // Same to `snappedThumbLocation` in `nsSliderFrame::CurrentPositionChanged`,
+  // to avoid putting the scroll thumb at subpixel positions which cause
+  // needless invalidations
+  nscoord appUnitsPerPixel = PresContext()->AppUnitsPerDevPixel();
+  thumbPos =
+      ToAppUnits(thumbPos.ToNearestPixels(appUnitsPerPixel), appUnitsPerPixel);
+
   const LogicalPoint logicalPos(wm, thumbPos, availSize);
   // TODO: It seems like a lot of this stuff should really belong in the thumb's
   // reflow code rather than here, but since we rely on the frame tree structure

testing/web-platform/meta/css/css-contain/content-visibility/content-visibility-075.html.ini

@@ -1,6 +1,4 @@
 [content-visibility-075.html]
   fuzzy:
-    # Tiny pixels differ around the scrollbar thumb.
-    if (os == "mac"): maxDifference=0-19;totalPixels=0-26
     # Tiny pixels differ around "P".
     if os == "win": maxDifference=0-47;totalPixels=0-2

testing/web-platform/meta/css/css-contain/content-visibility/content-visibility-076.html.ini

@@ -1,6 +1,4 @@
 [content-visibility-076.html]
   fuzzy:
-    # Tiny pixels differ around the scrollbar thumb.
-    if (os == "mac"): maxDifference=0-19;totalPixels=0-26
     # Tiny pixels differ around "P".
     if os == "win": maxDifference=0-47;totalPixels=0-2