Bug 1219296 - Move the layout.css.scroll-snap.proximity-threshold pref to gfxPrefs, so it can be queried on the compositor thread. r=kats

MozReview-Commit-ID: 6apoF0810Ea

--HG--
extra : rebase_source : 7860ad185b7c131ef19faa2befb3a09f75b7ad6e

gfx/thebes/gfxPrefs.h

@@ -398,6 +398,7 @@ private:
   DECL_GFX_PREF(Live, "layout.css.scroll-behavior.spring-constant", ScrollBehaviorSpringConstant, float, 250.0f);
   DECL_GFX_PREF(Live, "layout.css.scroll-snap.prediction-max-velocity", ScrollSnapPredictionMaxVelocity, int32_t, 2000);
   DECL_GFX_PREF(Live, "layout.css.scroll-snap.prediction-sensitivity", ScrollSnapPredictionSensitivity, float, 0.750f);
+  DECL_GFX_PREF(Live, "layout.css.scroll-snap.proximity-threshold", ScrollSnapProximityThreshold, int32_t, 200);
   DECL_GFX_PREF(Once, "layout.css.touch_action.enabled",       TouchActionEnabled, bool, false);
   DECL_GFX_PREF(Live, "layout.display-list.dump",              LayoutDumpDisplayList, bool, false);
   DECL_GFX_PREF(Live, "layout.event-regions.enabled",          LayoutEventRegionsEnabledDoNotUseDirectly, bool, false);

layout/generic/ScrollSnap.cpp

@@ -5,6 +5,7 @@
 
 #include "FrameMetrics.h"
 #include "ScrollSnap.h"
+#include "gfxPrefs.h"
 #include "mozilla/Maybe.h"
 #include "mozilla/Preferences.h"
 #include "nsLineLayout.h"
@@ -290,8 +291,7 @@ Maybe<nsPoint> ScrollSnapUtils::GetSnapPointForDestination(
   ProcessScrollSnapCoordinates(calcSnapPoints, aSnapInfo.mScrollSnapCoordinates, destPos);
   bool snapped = false;
   nsPoint finalPos = calcSnapPoints.GetBestEdge();
-  nscoord proximityThreshold =
-    Preferences::GetInt("layout.css.scroll-snap.proximity-threshold", 0);
+  nscoord proximityThreshold = gfxPrefs::ScrollSnapProximityThreshold();
   proximityThreshold = nsPresContext::CSSPixelsToAppUnits(proximityThreshold);
   if (aSnapInfo.mScrollSnapTypeY == NS_STYLE_SCROLL_SNAP_TYPE_PROXIMITY &&
       std::abs(aDestination.y - finalPos.y) > proximityThreshold) {