Pass clickEvent to controlElement inside label if targetNode can't start selection

third_party/WebKit/Source/core/html/HTMLLabelElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2010 Apple Inc. All rights reserved.
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 145 matching lines @@
     //       is clicked, then click event is passed to control element and
     //       control element is focused.
 
     bool is_label_text_selected = false;
 
     // If the click is not simulated and the text of the label element
     // is selected by dragging over it, then return without passing the
     // click event to control element.
     // Note: check if it is a MouseEvent because a click event may
     // not be an instance of a MouseEvent if created by document.createEvent().
-    if (evt->IsMouseEvent() && ToMouseEvent(evt)->HasPosition()) {
+    if (evt->IsMouseEvent() && ToMouseEvent(evt)->HasPosition() &&
+        isHTMLLabelElement(evt->target()->ToNode())) {

[yosin_UTC9, 2017/05/09 00:56:06]

I think we should check Event#srcElement in selection based on comment in the
code:
 // Check if there is a selection and click is not on the
 // selection.

[tkent, 2017/05/09 02:14:26]

Does this work if <label> has a child element?  e.g. <label><input
type=checkbox> <b><img ...> Check this</b></label>

[tanvir, 2017/05/09 12:02:32]

1) On using the label element as below

<label><input type=checkbox>
<b><img ...> Check this</b></label>

"If there is selection of label element by dragging, no click
event is passed. Also, no focus on control element"  is failing.
 
The reason is isHTMLLabelElement(evt->target()->ToNode()) fails in this case, as
evt->target()->ToNode() is a <B> element.

With PatchSet#3 i submitted this works fine.

2) There is one difference i observed with Firefox behaviour (Please check this
for reference https://jsfiddle.net/tanvirR/egcpLffh/4/ ),
When we select any text, and then click on <img> inside a Label, then control
gets focused and the selection gets cleared.
But in our case, the selection is not cleared. 

The reason as SelectionController::HandleSingleClick prevents it by the below
condition
  if (!(inner_node && inner_node->GetLayoutObject() &&
        mouse_down_may_start_select_))
    return false;
As on certain nodes canStartSelection returns false, like <img> Element.

This is the reason i added evt->target()->ToNode()->CanStartSelection() in my
PS#3 to by pass those
nodes on which CanStartSelection() can not be done.

[tanvir, 2017/05/10 13:44:36]

Hi Yosin, I couldn't get what you meant by " we should check Event#srcElement in
selection ", Did you meant at SelectionController ?

       if (LocalFrame* frame = GetDocument().GetFrame()) {
         // Check if there is a selection and click is not on the
         // selection.
         if (GetLayoutObject() && GetLayoutObject()->IsSelectable() &&
             frame->Selection()
                 .ComputeVisibleSelectionInDOMTreeDeprecated()
                 .IsRange() &&
             !frame->GetEventHandler()
                  .GetSelectionController()
                  .MouseDownWasSingleClickInSelection())
@@ skipping 53 matching lines @@
 }
 
 void HTMLLabelElement::AccessKeyAction(bool send_mouse_events) {
   if (HTMLElement* element = control())
     element->AccessKeyAction(send_mouse_events);
   else
     HTMLElement::AccessKeyAction(send_mouse_events);
 }
 
 }  // namespace blink