b=767710 don't let additional events with the same destination restart smooth scrolling r=mstange

--HG--
extra : transplant_source : _%BA%CE%F1A%B7%40N%14VMZ%1D%2C%98Lg%40%F7%AF

layout/generic/nsGfxScrollFrame.cpp

@@ -1345,15 +1345,23 @@ nsGfxScrollFrameInner::AsyncScroll::InitSmoothScroll(TimeStamp aTime,
                                                      nsPoint aDestination,
                                                      nsIAtom *aOrigin,
                                                      const nsRect& aRange) {
+  mRange = aRange;
+  TimeDuration duration = CalcDurationForEventTime(aTime, aOrigin);
   nsSize currentVelocity(0, 0);
   if (!mIsFirstIteration) {
+    // If an additional event has not changed the destination, then do not let
+    // another minimum duration reset slow things down.  If it would then
+    // instead continue with the existing timing function.
+    if (aDestination == mDestination &&
+        aTime + duration > mStartTime + mDuration)
+      return;
+
     currentVelocity = VelocityAt(aTime);
     mStartPos = PositionAt(aTime);
   }
   mStartTime = aTime;
-  mDuration = CalcDurationForEventTime(aTime, aOrigin);
+  mDuration = duration;
   mDestination = aDestination;
-  mRange = aRange;
   InitTimingFunction(mTimingFunctionX, mStartPos.x, currentVelocity.width,
                      aDestination.x);
   InitTimingFunction(mTimingFunctionY, mStartPos.y, currentVelocity.height,