Bug 1592435 - Don't allow a main thread scroll update to interrupt a touch-drag gesture. r=tnikkel

This makes the behaviour for touch-drags match the behaviour we already have
for pan gesture events.

Depends on D51201

Differential Revision: https://phabricator.services.mozilla.com/D51202

--HG--
extra : moz-landing-system : lando

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -4992,7 +4992,7 @@ bool AsyncPanZoomController::HasReadyTouchBlock() const {
 }
 
 bool AsyncPanZoomController::CanHandleScrollOffsetUpdate(PanZoomState aState) {
-  return aState == PAN_MOMENTUM;
+  return aState == PAN_MOMENTUM || IsPanningState(aState);
 }
 
 void AsyncPanZoomController::SetState(PanZoomState aNewState) {
@@ -5061,9 +5061,13 @@ bool AsyncPanZoomController::IsTransformingState(PanZoomState aState) {
   return !(aState == NOTHING || aState == TOUCHING);
 }
 
+bool AsyncPanZoomController::IsPanningState(PanZoomState aState) {
+  return (aState == PANNING || aState == PANNING_LOCKED_X ||
+          aState == PANNING_LOCKED_Y);
+}
+
 bool AsyncPanZoomController::IsInPanningState() const {
-  return (mState == PANNING || mState == PANNING_LOCKED_X ||
-          mState == PANNING_LOCKED_Y);
+  return IsPanningState(mState);
 }
 
 void AsyncPanZoomController::UpdateZoomConstraints(

gfx/layers/apz/src/AsyncPanZoomController.h

@@ -1229,6 +1229,8 @@ class AsyncPanZoomController {
   // held whenever this is updated. In practice though... see bug 897017.
   PanZoomState mState;
 
+  static bool IsPanningState(PanZoomState aState);
+
   /**
    * Returns whether the specified PanZoomState does not need to be reset when
    * a scroll offset update is processed.