Bug 1345355 - Allow pinch gestures with a zero span change but a nonzero focus change to scroll. r=botond

It appears that some touchpad devices send us "touch" events (i.e. WM_TOUCH on
Windows) but with all touch points having the same coordinates. This ends up
getting detected as a zero-span pinch gesture in APZ, which short-circuits early
and doesn't really get processed. Therefore even if the focus point changes we
don't do any corresponding scroll. This patch shifts things around a little so
that the short-circuit doesn't happen quite so early, and we still scroll when
the focus point changes, even if the span is zero.

MozReview-Commit-ID: 3CaQN1MsM8y

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -1368,14 +1368,6 @@ nsEventStatus AsyncPanZoomController::OnScale(const PinchGestureInput& aEvent) {
   MOZ_ASSERT(mFrameMetrics.IsRootContent());
   MOZ_ASSERT(mFrameMetrics.GetZoom().AreScalesSame());
 
-  float prevSpan = aEvent.mPreviousSpan;
-  if (fabsf(prevSpan) <= EPSILON || fabsf(aEvent.mCurrentSpan) <= EPSILON) {
-    // We're still handling it; we've just decided to throw this event away.
-    return nsEventStatus_eConsumeNoDefault;
-  }
-
-  float spanRatio = aEvent.mCurrentSpan / aEvent.mPreviousSpan;
-
   {
     ReentrantMonitorAutoEnter lock(mMonitor);
 
@@ -1384,12 +1376,29 @@ nsEventStatus AsyncPanZoomController::OnScale(const PinchGestureInput& aEvent) {
     CSSPoint cssFocusPoint = focusPoint / mFrameMetrics.GetZoom();
 
     ParentLayerPoint focusChange = mLastZoomFocus - focusPoint;
+    mLastZoomFocus = focusPoint;
     // If displacing by the change in focus point will take us off page bounds,
     // then reduce the displacement such that it doesn't.
     focusChange.x -= mX.DisplacementWillOverscrollAmount(focusChange.x);
     focusChange.y -= mY.DisplacementWillOverscrollAmount(focusChange.y);
     ScrollBy(focusChange / userZoom);
 
+    // If the span is zero or close to it, we don't want to process this zoom
+    // change because we're going to get wonky numbers for the spanRatio. So
+    // let's bail out here. Note that we do this after the focus-change-scroll
+    // above, so that if we have a pinch with zero span but changing focus,
+    // such as generated by some Synaptics touchpads on Windows, we still
+    // scroll properly.
+    float prevSpan = aEvent.mPreviousSpan;
+    if (fabsf(prevSpan) <= EPSILON || fabsf(aEvent.mCurrentSpan) <= EPSILON) {
+      // We might have done a nonzero ScrollBy above, so update metrics and
+      // repaint/recomposite
+      ScheduleCompositeAndMaybeRepaint();
+      UpdateSharedCompositorFrameMetrics();
+      return nsEventStatus_eConsumeNoDefault;
+    }
+    float spanRatio = aEvent.mCurrentSpan / aEvent.mPreviousSpan;
+
     // When we zoom in with focus, we can zoom too much towards the boundaries
     // that we actually go over them. These are the needed displacements along
     // either axis such that we don't overscroll the boundaries when zooming.
@@ -1448,8 +1457,6 @@ nsEventStatus AsyncPanZoomController::OnScale(const PinchGestureInput& aEvent) {
     // We did a ScrollBy call above even if we didn't do a scale, so we
     // should composite for that.
     ScheduleComposite();
-
-    mLastZoomFocus = focusPoint;
   }
 
   return nsEventStatus_eConsumeNoDefault;