Lock element at start of gesture sequence.

Source/core/page/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 337 matching lines @@
     m_previousGestureScrolledNode = 0;
     m_scrollbarHandlingScrollGesture = 0;
     m_maxMouseMovedDuration = 0;
     m_baseEventType = PlatformEvent::NoType;
     m_didStartDrag = false;
     m_touchPressed = false;
     m_mouseDownMayStartSelect = false;
     m_mouseDownMayStartDrag = false;
     m_lastShowPressTimestamp = 0;
     m_lastDeferredTapElement = 0;
+    m_elementHandlingGestureSequence = 0;
 }
 
 void EventHandler::nodeWillBeRemoved(Node& nodeToBeRemoved)
 {
     if (!nodeToBeRemoved.containsIncludingShadowDOM(m_clickNode.get()))
         return;
     if (nodeToBeRemoved.isInShadowTree()) {
         m_clickNode = nodeToBeRemoved.parentOrShadowHostNode();
     } else {
         // We don't dispatch click events if the mousedown node is removed
@@ skipping 1929 matching lines @@
         // of the active state so that it's visible to the user for at least one frame.
         activeInterval = WTF::currentTime() - m_lastShowPressTimestamp;
         shouldKeepActiveForMinInterval = m_lastShowPressTimestamp && activeInterval < minimumActiveInterval;
         if (shouldKeepActiveForMinInterval)
             hitType |= HitTestRequest::ReadOnly;
     }
     else
         hitType |= HitTestRequest::Active | HitTestRequest::ReadOnly;
 
     if ((!scrollbar && !eventTarget) || !(hitType & HitTestRequest::ReadOnly)) {
-        IntPoint hitTestPoint = m_frame->view()->windowToContents(adjustedPoint);
+        IntPoint hitTestPoint;
+        // FIXME: Track element across iframes when out of process iframes have more structure.
+        if (m_elementHandlingGestureSequence && gestureEvent.type() != PlatformEvent::GestureTapDown) {
+            Element* gestureElement = m_elementHandlingGestureSequence.get();

[Rick Byers, 2014/02/21 03:46:54]

I don't think this handles all the cases (or even the most likely cases).  Eg.
what if tapDown causes a new element to appear over top of the element.  I think
we decided we want the tap to still go to the original (i.e. highlighted)
element, even though a hit test at it's co-ordinates would now reach the element
on top.

So I think you need to find a way to skip the hit test entirely and used the
cached element.

+            hitTestPoint.setX((gestureElement->offsetLeft() + gestureElement->offsetWidth()) / 2);
+            hitTestPoint.setY((gestureElement->offsetTop() + gestureElement->offsetHeight()) / 2);
+        } else {
+            hitTestPoint = m_frame->view()->windowToContents(adjustedPoint);
+        }
+
         HitTestResult result = hitTestResultAtPoint(hitTestPoint, hitType | HitTestRequest::AllowFrameScrollbars);
 
+        if (gestureEvent.type() == PlatformEvent::GestureTapDown)
+            m_elementHandlingGestureSequence = result.innerElement();

[Rick Byers, 2014/02/21 03:46:54]

you probably don't want to hold on to this indefinitely (could keep an expensive
tree of objects alive for a long time), but it's not obvious to know when to
release it.  The simple case is on Tap and GestureScrollEnd, but I think there
are some more subtle cases (eg. multi-finger swipe).

Perhaps you should just treat a tap sequence (TapDown...Tap/TapCancel) and a
scroll sequence (ScrollBegin...ScrollEnd) as independent sequences.  I guess
this boils down to how we want things to behave if ShowPress triggers a
scrollable element to appear on top - does pressing then scrolling scroll the
new element or the background one?  It would be valuable to know the behavior of
different browsers here.

Worst case we could add a GestureEnd.

+
         if (shouldKeepActiveForMinInterval) {
             m_lastDeferredTapElement = result.innerElement();
             m_activeIntervalTimer.startOneShot(minimumActiveInterval - activeInterval);
         }
 
         eventTarget = result.targetNode();
         if (!scrollbar) {
             FrameView* view = m_frame->view();
             scrollbar = view ? view->scrollbarAtPoint(gestureEvent.position()) : 0;
         }
         if (!scrollbar)
             scrollbar = result.scrollbar();
     }
 
     if (scrollbar) {
+        // If finger was on scrollbar, it will be locked by the scroll handling code.
+        m_elementHandlingGestureSequence = 0;
         bool eventSwallowed = scrollbar->gestureEvent(gestureEvent);
         if (gestureEvent.type() == PlatformEvent::GestureTapDown && eventSwallowed) {
             m_scrollbarHandlingScrollGesture = scrollbar;
         } else if (gestureEvent.type() == PlatformEvent::GestureScrollEnd
             || gestureEvent.type() == PlatformEvent::GestureFlingStart
             || !eventSwallowed) {
             m_scrollbarHandlingScrollGesture = 0;
         }
 
         if (eventSwallowed)
@@ skipping 1673 matching lines @@
 unsigned EventHandler::accessKeyModifiers()
 {
 #if OS(MACOSX)
     return PlatformEvent::CtrlKey | PlatformEvent::AltKey;
 #else
     return PlatformEvent::AltKey;
 #endif
 }
 
 } // namespace WebCore