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

gfx/layers/apz/test/reftest/reftest.list

@@ -29,7 +29,7 @@ fuzzy-if(Android,0-31,0-29) fuzzy-if(gtkWidget,0-18,0-49) fuzzy-if(cocoaWidget,0
 # clipped away.
 fuzzy-if(Android,0-54,0-22) fuzzy-if(!Android,0-128,0-137) pref(apz.allow_zooming,true) == root-scrollbar-zoomed-in.html root-scrollbar-zoomed-in-ref.html
 fuzzy-if(Android,0-54,0-22) fuzzy-if(!Android,0-128,0-137) pref(apz.allow_zooming,true) pref(apz.allow_zooming_out,true) == root-scrollbar-zoomed-out.html root-scrollbar-zoomed-out-ref.html
-fuzzy-if(Android,0-54,0-27) fuzzy-if(!Android,0-128,0-137) pref(apz.allow_zooming,true) == root-scrollbar-zoomed-in-async-scroll.html root-scrollbar-zoomed-in-ref.html
+fuzzy-if(Android,0-54,0-31) fuzzy-if(!Android,0-128,0-137) pref(apz.allow_zooming,true) == root-scrollbar-zoomed-in-async-scroll.html root-scrollbar-zoomed-in-ref.html
 fuzzy-if(Android,0-54,0-25) fuzzy-if(!Android,0-128,0-137) pref(apz.allow_zooming,true) pref(apz.allow_zooming_out,true) == root-scrollbar-zoomed-out-async-scroll.html root-scrollbar-zoomed-out-ref.html
 fuzzy-if(Android,0-51,0-50) fuzzy-if(!Android,0-128,0-137) pref(apz.allow_zooming,true) == subframe-scrollbar-zoomed-in-async-scroll.html subframe-scrollbar-zoomed-in-async-scroll-ref.html
 fuzzy-if(Android,0-28,0-23) fuzzy-if(!Android,0-107,0-34) pref(apz.allow_zooming,true) pref(apz.allow_zooming_out,true) == subframe-scrollbar-zoomed-out-async-scroll.html subframe-scrollbar-zoomed-out-async-scroll-ref.html

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

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

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

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