Bug 833795 - Sample offset on touch start. r=drs

content/events/src/nsEventStateManager.cpp

@@ -1525,29 +1525,32 @@ nsEventStateManager::IsRemoteTarget(nsIContent* target) {
   return false;
 }
 
-/*static*/ void
-nsEventStateManager::MapEventCoordinatesForChildProcess(nsFrameLoader* aFrameLoader,
-                                                        nsEvent* aEvent)
+/*static*/ nsIntPoint
+nsEventStateManager::GetChildProcessOffset(nsFrameLoader* aFrameLoader,
+                                           const nsEvent& aEvent)
 {
   // The "toplevel widget" in child processes is always at position
   // 0,0.  Map the event coordinates to match that.
   nsIFrame* targetFrame = aFrameLoader->GetPrimaryFrameOfOwningContent();
   if (!targetFrame) {
-    return;
+    return nsIntPoint();
   }
   nsPresContext* presContext = targetFrame->PresContext();
 
+  // Find out how far we're offset from the nearest widget.
+  nsPoint pt = nsLayoutUtils::GetEventCoordinatesRelativeTo(&aEvent,
+                                                            targetFrame);
+  return pt.ToNearestPixels(presContext->AppUnitsPerDevPixel());
+}
+
+/*static*/ void
+nsEventStateManager::MapEventCoordinatesForChildProcess(
+  const nsIntPoint& aOffset, nsEvent* aEvent)
+{
   if (aEvent->eventStructType != NS_TOUCH_EVENT) {
-    nsPoint pt = nsLayoutUtils::GetEventCoordinatesRelativeTo(aEvent,
-                                                              targetFrame);
-    aEvent->refPoint = pt.ToNearestPixels(presContext->AppUnitsPerDevPixel());
+    aEvent->refPoint = aOffset;
   } else {
     aEvent->refPoint = nsIntPoint();
-    // Find out how far we're offset from the nearest widget.
-    nsPoint offset =
-      nsLayoutUtils::GetEventCoordinatesRelativeTo(aEvent, targetFrame);
-    nsIntPoint intOffset =
-      offset.ToNearestPixels(presContext->AppUnitsPerDevPixel());
     nsTouchEvent* touchEvent = static_cast<nsTouchEvent*>(aEvent);
     // Then offset all the touch points by that distance, to put them
     // in the space where top-left is 0,0.
@@ -1555,12 +1558,20 @@ nsEventStateManager::MapEventCoordinatesForChildProcess(nsFrameLoader* aFrameLoa
     for (uint32_t i = 0; i < touches.Length(); ++i) {
       nsIDOMTouch* touch = touches[i];
       if (touch) {
-        touch->mRefPoint += intOffset;
+        touch->mRefPoint += aOffset;
       }
     }
   }
 }
 
+/*static*/ void
+nsEventStateManager::MapEventCoordinatesForChildProcess(nsFrameLoader* aFrameLoader,
+                                                        nsEvent* aEvent)
+{
+  nsIntPoint offset = GetChildProcessOffset(aFrameLoader, *aEvent);
+  MapEventCoordinatesForChildProcess(offset, aEvent);
+}
+
 bool
 CrossProcessSafeEvent(const nsEvent& aEvent)
 {

content/events/src/nsEventStateManager.h

@@ -192,10 +192,15 @@ public:
   static void SetFullScreenState(mozilla::dom::Element* aElement, bool aIsFullScreen);
 
   static bool IsRemoteTarget(nsIContent* aTarget);
+  static nsIntPoint GetChildProcessOffset(nsFrameLoader* aFrameLoader,
+                                          const nsEvent& aEvent);
 
   static void MapEventCoordinatesForChildProcess(nsFrameLoader* aFrameLoader,
                                                  nsEvent* aEvent);
 
+  static void MapEventCoordinatesForChildProcess(const nsIntPoint& aOffset,
+                                                 nsEvent* aEvent);
+
   // Holds the point in screen coords that a mouse event was dispatched to,
   // before we went into pointer lock mode. This is constantly updated while
   // the pointer is not locked, but we don't update it while the pointer is

dom/ipc/TabParent.cpp

@@ -636,6 +636,18 @@ bool TabParent::SendRealTouchEvent(nsTouchEvent& event)
     return false;
   }
   if (event.message == NS_TOUCH_START) {
+    // Adjust the widget coordinates to be relative to our frame.
+    nsRefPtr<nsFrameLoader> frameLoader = GetFrameLoader();
+    if (!frameLoader) {
+      // No frame anymore?
+      sEventCapturer = nullptr;
+      return false;
+    }
+
+    mChildProcessOffsetAtTouchStart =
+        nsEventStateManager::GetChildProcessOffset(frameLoader,
+                                                   event);
+
     MOZ_ASSERT((!sEventCapturer && mEventCaptureDepth == 0) ||
                (sEventCapturer == this && mEventCaptureDepth > 0));
     // We want to capture all remaining touch events in this series
@@ -693,16 +705,8 @@ TabParent::TryCapture(const nsGUIEvent& aEvent)
     return false;
   }
 
-  // Adjust the widget coordinates to be relative to our frame.
-  nsRefPtr<nsFrameLoader> frameLoader = GetFrameLoader();
-
-  if (!frameLoader) {
-    // No frame anymore?
-    sEventCapturer = nullptr;
-    return false;
-  }
-
-  nsEventStateManager::MapEventCoordinatesForChildProcess(frameLoader, &event);
+  nsEventStateManager::MapEventCoordinatesForChildProcess(
+    mChildProcessOffsetAtTouchStart, &event);
 
   SendRealTouchEvent(event);
   return true;

dom/ipc/TabParent.h

@@ -313,6 +313,11 @@ private:
     // dispatch to content.
     void MaybeForwardEventToRenderFrame(const nsInputEvent& aEvent,
                                         nsInputEvent* aOutEvent);
+    // The offset for the child process which is sampled at touch start. This
+    // means that the touch events are relative to where the frame was at the
+    // start of the touch. We need to look for a better solution to this
+    // problem see bug 872911.
+    nsIntPoint mChildProcessOffsetAtTouchStart;
     // When true, we've initiated normal shutdown and notified our
     // managing PContent.
     bool mMarkedDestroying;