Do not send redundant selectionchange-events (decouple focus)

third_party/WebKit/Source/core/page/FocusController.cpp

 /*
  * Copyright (C) 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nuanti Ltd.
  *
  * 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.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 1058 matching lines @@
 
   FrameSelection& selection = oldFocusedFrame->selection();
   if (selection.isNone())
     return;
 
   Node* selectionStartNode = selection.selection().start().anchorNode();
   if (selectionStartNode == newFocusedElement ||
       selectionStartNode->isDescendantOf(newFocusedElement))
     return;
 
+  if (newFocusedElement && newFocusedElement->isTextControl())

[yosin_UTC9, 2017/01/05 05:35:33]

Should we also bail out for content editable?

Rather than adding more condition, I think it is better to get rid of this
function, clearSelecitonIfNeeded()
and ask FrameSelection to clear selection if new focus element can't have
selection.

[hugoh_UTC2, 2017/01/06 17:31:44]

Yes, you're right. When focus jumps from <input> to <p contenteditable> I see
that Blink calls into didChangeSelection twice. I added another condition for
this and a unit test that also checks this. 
You mean clear the selection if the new focused element _can_ have a selection?
Or, does it matter? Don't we always want to clear the selection when focus
moves?

+    return;
+
   if (!enclosingTextControl(selectionStartNode))
     return;
 
   if (selectionStartNode->isInShadowTree() &&
       selectionStartNode->ownerShadowHost() == newFocusedElement)
     return;
 
   selection.clear();
 }
 
@@ skipping 304 matching lines @@
 
   return consumed;
 }
 
 DEFINE_TRACE(FocusController) {
   visitor->trace(m_page);
   visitor->trace(m_focusedFrame);
 }
 
 }  // namespace blink