Convert main-thread overscroll parameters into viewport space.

third_party/WebKit/Source/core/input/EventHandler.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2012 Digia Plc. and/or its subsidiary(-ies)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 2355 matching lines @@
 static inline FloatSize adjustOverscroll(FloatSize unusedDelta)
 {
     if (std::abs(unusedDelta.width()) < minimumOverscrollDelta)
         unusedDelta.setWidth(0);
     if (std::abs(unusedDelta.height()) < minimumOverscrollDelta)
         unusedDelta.setHeight(0);
 
     return unusedDelta;
 }
 
-void EventHandler::handleOverscroll(const ScrollResult& scrollResult, const FloatPoint& position, const FloatSize& velocity)
+void EventHandler::handleOverscroll(const ScrollResult& scrollResult, const FloatPoint& positionInRootFrame, const FloatSize& velocity)

[majidvp, 2016/04/13 18:20:38]

s/velocity/velocityInRootFram/

[bokan, 2016/04/13 19:43:49]

Done.

 {
     ASSERT(m_frame->isMainFrame());
+    VisualViewport& visualViewport = m_frame->page()->frameHost().visualViewport();
 
     FloatSize unusedDelta(scrollResult.unusedScrollDeltaX, scrollResult.unusedScrollDeltaY);
     unusedDelta = adjustOverscroll(unusedDelta);
+
+    // The incoming event had its delta, position, and velocity transformed by
+    // the visual viewport transform so when we pass it back we should undo it
+    // to go back to viewport space.

[majidvp, 2016/04/13 18:20:38]

Can we simplify this comment by referencing from->to coordinate spaces e.g., 

translate  delta, position and velocity of the incoming event from root frame
coordinate space to viewport coordinate space before reporting.

[bokan, 2016/04/13 19:43:49]

Looking at this again, this comment isn't adding much, it's clear we're going
from root frame->viewport by looking at the code so I've removed the comment.
Let me know if you disagree.

+    FloatSize deltaInViewport = unusedDelta.scaledBy(visualViewport.scale());
+    FloatSize velocityInViewport = velocity.scaledBy(visualViewport.scale());
+    FloatPoint positionInViewport =
+        visualViewport.rootFrameToViewport(positionInRootFrame);
+
     resetOverscroll(scrollResult.didScrollX, scrollResult.didScrollY);
-    if (unusedDelta != FloatSize()) {
-        m_accumulatedRootOverscroll += unusedDelta;
-        m_frame->chromeClient().didOverscroll(unusedDelta, m_accumulatedRootOverscroll, position, velocity);
+    if (deltaInViewport != FloatSize()) {
+        m_accumulatedRootOverscroll += deltaInViewport;
+        m_frame->chromeClient().didOverscroll(deltaInViewport, m_accumulatedRootOverscroll, positionInViewport, velocityInViewport);
     }
 }
 
 WebInputEventResult EventHandler::handleGestureScrollUpdate(const PlatformGestureEvent& gestureEvent)
 {
     ASSERT(gestureEvent.type() == PlatformEvent::GestureScrollUpdate);
 
     // Negate the deltas since the gesture event stores finger movement and
     // scrolling occurs in the direction opposite the finger's movement
     // direction. e.g. Finger moving up has negative event delta but causes the
@@ skipping 60 matching lines @@
             if (gestureEvent.preventPropagation())
                 stopNode = m_previousGestureScrolledNode.get();
 
             bool consumed = false;
             ScrollResult result = physicalScroll(granularity, delta, node, &stopNode, &consumed);
 
             if (gestureEvent.preventPropagation())
                 m_previousGestureScrolledNode = stopNode;
 
             if (m_frame->isMainFrame() && (!stopNode || stopNode->layoutObject() == m_frame->view()->layoutView())) {
-                FloatPoint position = FloatPoint(gestureEvent.position().x(), gestureEvent.position().y());
-                handleOverscroll(result, position, velocity);
+                FloatPoint positionInRootFrame = FloatPoint(gestureEvent.position().x(), gestureEvent.position().y());
+                handleOverscroll(result, positionInRootFrame, velocity);
             } else {
                 resetOverscroll(result.didScrollX, result.didScrollY);
             }
 
             if (consumed)
                 return WebInputEventResult::HandledSystem;
         }
     }
 
     return WebInputEventResult::NotHandled;
@@ skipping 1535 matching lines @@
 PlatformEvent::Modifiers EventHandler::accessKeyModifiers()
 {
 #if OS(MACOSX)
     return static_cast<PlatformEvent::Modifiers>(PlatformEvent::CtrlKey | PlatformEvent::AltKey);
 #else
     return PlatformEvent::AltKey;
 #endif
 }
 
 } // namespace blink