Bug 1519553 - Don't round scroll anchor adjustments to device pixels. r=dholbert

Differential Revision: https://phabricator.services.mozilla.com/D19107

--HG--
extra : source : 7f29bb7519563e3aaafa2e484a356409ce05aab7
extra : histedit_source : 3f1c287cc5984bf896ed43cccd7533d39cda40c9

layout/generic/ScrollAnchorContainer.cpp

@@ -345,15 +345,13 @@ void ScrollAnchorContainer::ApplyAdjustments() {
       break;
     }
   }
-  nsIntPoint physicalDevicePixels = physicalAdjustment.ToNearestPixels(
-      Frame()->PresContext()->AppUnitsPerDevPixel());
 
   MOZ_ASSERT(!mApplyingAnchorAdjustment);
   // We should use AutoRestore here, but that doesn't work with bitfields
   mApplyingAnchorAdjustment = true;
-  mScrollFrame->ScrollBy(
-      physicalDevicePixels, nsIScrollableFrame::DEVICE_PIXELS,
-      nsIScrollableFrame::INSTANT, nullptr, nsGkAtoms::relative);
+  mScrollFrame->ScrollTo(
+      mScrollFrame->GetScrollPosition() + physicalAdjustment,
+      nsIScrollableFrame::INSTANT, nsGkAtoms::relative);
   mApplyingAnchorAdjustment = false;
 
   nsPresContext* pc = Frame()->PresContext();