Report accurate Overscroll on handleWheel.

Source/platform/scroll/ScrollAnimator.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 74 matching lines @@
     float deltaY = canScrollY ? e.deltaY() : 0;
 
     ScrollResult result;
 
 #if !OS(MACOSX)
     ScrollGranularity granularity = e.hasPreciseScrollingDeltas() ? ScrollByPrecisePixel : ScrollByPixel;
 #else
     ScrollGranularity granularity = ScrollByPixel;
 #endif
 
-    IntSize maxForwardScrollDelta = m_scrollableArea->maximumScrollPosition() - m_scrollableArea->scrollPosition();
-    IntSize maxBackwardScrollDelta = m_scrollableArea->scrollPosition() - m_scrollableArea->minimumScrollPosition();
-    if ((deltaX < 0 && maxForwardScrollDelta.width() > 0)
-        || (deltaX > 0 && maxBackwardScrollDelta.width() > 0))
-        result.didScrollX = true;
-    if ((deltaY < 0 && maxForwardScrollDelta.height() > 0)
-        || (deltaY > 0 && maxBackwardScrollDelta.height() > 0))
-        result.didScrollY = true;
-    if (result.didScroll()) {
-        if (deltaY) {
-            if (e.granularity() == ScrollByPageWheelEvent) {
-                bool negative = deltaY < 0;
-                deltaY = m_scrollableArea->pageStep(VerticalScrollbar);
-                if (negative)
-                    deltaY = -deltaY;
-            }
-
-            ScrollResultOneDimensional resultY = userScroll(
-                VerticalScrollbar, granularity, m_scrollableArea->pixelStep(VerticalScrollbar), -deltaY);
-
-            if (e.granularity() != ScrollByPageWheelEvent) {
-                if (resultY.didScroll)
-                    result.unusedScrollDeltaY = -resultY.unusedScrollDelta;
-                else
-                    result.unusedScrollDeltaY = deltaY;
-            }
+    IntPoint currentPos = m_scrollableArea->scrollPosition();

[bokan, 2015/06/25 14:54:35]

Sorry, I (slightly) misled you before. We can't just compare the position since
if we use smooth scrolling (i.e. animated scroll) then we wont see a difference.
I think that's why the old code was checking against the max/min position.

But actually, I think we can remove this altogether since the userScroll calls
below will return true/false in the ScrollResultOneDimensional.didScroll. Just
fill the result.didScrollX/Y values from that.

[MuVen, 2015/06/25 17:05:35]

Done.

+    if (deltaY) {
+        if (e.granularity() == ScrollByPageWheelEvent) {
+            bool negative = deltaY < 0;
+            deltaY = m_scrollableArea->pageStep(VerticalScrollbar);
+            if (negative)
+                deltaY = -deltaY;
         }
 
-        if (deltaX) {
-            if (e.granularity() == ScrollByPageWheelEvent) {
-                bool negative = deltaX < 0;
-                deltaX = m_scrollableArea->pageStep(HorizontalScrollbar);
-                if (negative)
-                    deltaX = -deltaX;
-            }
+        ScrollResultOneDimensional resultY = userScroll(
+            VerticalScrollbar, granularity, m_scrollableArea->pixelStep(VerticalScrollbar), -deltaY);
 
-            ScrollResultOneDimensional resultX = userScroll(
-                HorizontalScrollbar, granularity, m_scrollableArea->pixelStep(HorizontalScrollbar), -deltaX);
-
-            if (e.granularity() != ScrollByPageWheelEvent) {
-                if (resultX.didScroll)
-                    result.unusedScrollDeltaX = -resultX.unusedScrollDelta;
-                else
-                    result.unusedScrollDeltaX = deltaX;
-            }
+        if (e.granularity() != ScrollByPageWheelEvent) {
+            if (resultY.didScroll)
+                result.unusedScrollDeltaY = -resultY.unusedScrollDelta;
+            else
+                result.unusedScrollDeltaY = deltaY;
         }
     }
 
+    if (deltaX) {
+        if (e.granularity() == ScrollByPageWheelEvent) {
+            bool negative = deltaX < 0;
+            deltaX = m_scrollableArea->pageStep(HorizontalScrollbar);
+            if (negative)
+                deltaX = -deltaX;
+        }
+
+        ScrollResultOneDimensional resultX = userScroll(
+            HorizontalScrollbar, granularity, m_scrollableArea->pixelStep(HorizontalScrollbar), -deltaX);
+
+        if (e.granularity() != ScrollByPageWheelEvent) {
+            if (resultX.didScroll)
+                result.unusedScrollDeltaX = -resultX.unusedScrollDelta;
+            else
+                result.unusedScrollDeltaX = deltaX;
+        }
+    }
+
+    IntPoint newPos = m_scrollableArea->scrollPosition();
+    result.didScrollX = newPos.x() != currentPos.x();
+    result.didScrollY = newPos.y() != currentPos.y();
     return result;
 }
 
 void ScrollAnimator::setCurrentPosition(const FloatPoint& position)
 {
     m_currentPosX = position.x();
     m_currentPosY = position.y();
 }
 
 FloatPoint ScrollAnimator::currentPosition() const
 {
     return FloatPoint(m_currentPosX, m_currentPosY);
 }
 
 void ScrollAnimator::notifyPositionChanged()
 {
     m_scrollableArea->scrollPositionChanged(DoublePoint(m_currentPosX, m_currentPosY), UserScroll);
 }
 
 float ScrollAnimator::clampScrollPosition(ScrollbarOrientation orientation, float pos)
 {
     float maxScrollPos = m_scrollableArea->maximumScrollPosition(orientation);
     float minScrollPos = m_scrollableArea->minimumScrollPosition(orientation);
     return std::max(std::min(pos, maxScrollPos), minScrollPos);
 }
 
 } // namespace blink