Update touch selection to only modify one selection point at a time.

content/browser/renderer_host/input/input_router_impl.cc

Index: content/browser/renderer_host/input/input_router_impl.cc
diff --git a/content/browser/renderer_host/input/input_router_impl.cc b/content/browser/renderer_host/input/input_router_impl.cc
index 441f9384bb8745e05efd53d9e1e645aa94255124..f3b4169705adf6b47ef7dc10f7bd2086f81edd7d 100644
--- a/content/browser/renderer_host/input/input_router_impl.cc
+++ b/content/browser/renderer_host/input/input_router_impl.cc
@@ -69,6 +69,7 @@ InputRouterImpl::InputRouterImpl(IPC::Sender* sender,
       ack_handler_(ack_handler),
       routing_id_(routing_id),
       select_range_pending_(false),
+      move_range_selection_extent_pending_(false),
       move_caret_pending_(false),
       mouse_move_pending_(false),
       mouse_wheel_pending_(false),
@@ -96,6 +97,8 @@ bool InputRouterImpl::SendInput(scoped_ptr<IPC::Message> message) {
     // Check for types that require an ACK.
     case InputMsg_SelectRange::ID:
       return SendSelectRange(message.Pass());
+    case InputMsg_MoveRangeSelectionExtent::ID:
+      return SendMoveRangeSelectionExtent(message.Pass());
     case InputMsg_MoveCaret::ID:
       return SendMoveCaret(message.Pass());
     case InputMsg_HandleInputEvent::ID:
@@ -267,8 +270,10 @@ bool InputRouterImpl::OnMessageReceived(const IPC::Message& message) {
   IPC_BEGIN_MESSAGE_MAP(InputRouterImpl, message)
     IPC_MESSAGE_HANDLER(InputHostMsg_HandleInputEvent_ACK, OnInputEventAck)
     IPC_MESSAGE_HANDLER(InputHostMsg_DidOverscroll, OnDidOverscroll)
-    IPC_MESSAGE_HANDLER(ViewHostMsg_MoveCaret_ACK, OnMsgMoveCaretAck)
-    IPC_MESSAGE_HANDLER(ViewHostMsg_SelectRange_ACK, OnSelectRangeAck)
+    IPC_MESSAGE_HANDLER(InputHostMsg_MoveCaret_ACK, OnMsgMoveCaretAck)
+    IPC_MESSAGE_HANDLER(InputHostMsg_SelectRange_ACK, OnSelectRangeAck)
+    IPC_MESSAGE_HANDLER(InputHostMsg_MoveRangeSelectionExtent_ACK,
+                        OnMoveRangeSelectionExtentAck)
     IPC_MESSAGE_HANDLER(ViewHostMsg_HasTouchEventHandlers,
                         OnHasTouchEventHandlers)
     IPC_MESSAGE_HANDLER(InputHostMsg_SetTouchAction,
@@ -309,6 +314,20 @@ bool InputRouterImpl::SendSelectRange(scoped_ptr<IPC::Message> message) {
   return Send(message.release());
 }
 
+bool InputRouterImpl::SendMoveRangeSelectionExtent(
+    scoped_ptr<IPC::Message> message) {
+  DCHECK(message->type() == InputMsg_MoveRangeSelectionExtent::ID);

[jdduke (slow), 2014/10/21 16:57:36]

Hmm, so, in theory |SelectRange| and |MoveRangeSelectionExtent| messages could
be interleaved, arriving out-of-order in the render process (e.g., if a
SelectRange is pending and we get a new MoveRangeSelectionExtent message).

The simplest solution is probably: 

1) Keep a unified queue of IPCs for both messages types.
2) If the tail message of the queue is the same type as the new IPC, replace the
tail with the new IPC. Otherwise, enqueue it.
3) When an ack is received, dispatch the head of the queue.

Thoughts? Sorry I didn't think of this sooner, but hopefully it's not too crazy
to implement. Unfortunately, I don't think there's a ScopedDeque in base/ yet.

If that's confusing, feel free to TODO(jdduke) it and I'll implement it in a
follow-up patch.

[christiank, 2014/10/22 12:16:06]

I think you're right. Maybe we could use the queue for caret messages as well?
While there is no dependency between selection and caret messages it would be
possible to remove some code (and the other TODO that I added).

I'll attempt a fix for this using a std::deque. Unfortunately I suspect some
manual memory management will be required.

+  // TODO(jdduke): Factor out common logic between selection and caret-related
+  //               IPC messages.
+  if (move_range_selection_extent_pending_) {
+    next_range_selection_extent_move_ = message.Pass();
+    return true;
+  }
+
+  move_range_selection_extent_pending_ = true;
+  return Send(message.release());
+}
+
 bool InputRouterImpl::SendMoveCaret(scoped_ptr<IPC::Message> message) {
   DCHECK(message->type() == InputMsg_MoveCaret::ID);
   if (move_caret_pending_) {
@@ -493,6 +512,12 @@ void InputRouterImpl::OnSelectRangeAck() {
     SendSelectRange(next_selection_range_.Pass());
 }
 
+void InputRouterImpl::OnMoveRangeSelectionExtentAck() {
+  move_range_selection_extent_pending_ = false;
+  if (next_range_selection_extent_move_)
+    SendMoveRangeSelectionExtent(next_range_selection_extent_move_.Pass());
+}
+
 void InputRouterImpl::OnHasTouchEventHandlers(bool has_handlers) {
   TRACE_EVENT1("input", "InputRouterImpl::OnHasTouchEventHandlers",
                "has_handlers", has_handlers);
@@ -680,6 +705,7 @@ bool InputRouterImpl::HasPendingEvents() const {
          mouse_move_pending_ ||
          mouse_wheel_pending_ ||
          select_range_pending_ ||
+         move_range_selection_extent_pending_ ||
          move_caret_pending_;
 }