Accept text input only if selection has focus.

third_party/WebKit/Source/core/editing/SelectionController.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)
  * Copyright (C) 2015 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
@@ skipping 217 matching lines @@
   }
 
   if (selection_state_ == SelectionState::kExtendedSelection) {
     UpdateSelectionForMouseDownDispatchingSelectStart(
         inner_node, selection, kCharacterGranularity,
         HandleVisibility::kNotVisible);
     return false;
   }
 
   if (visible_pos.IsNull()) {
-    UpdateSelectionForMouseDownDispatchingSelectStart(
-        inner_node, VisibleSelectionInFlatTree(), kCharacterGranularity,
-        HandleVisibility::kNotVisible);
+    Selection().SetSelection(SelectionInFlatTree());

[yosin_UTC9, 2017/05/17 10:26:02]

Do other browser clear selection at clicking user-select:none?

     return false;
   }
 
   bool is_handle_visible = false;
   if (HasEditableStyle(*inner_node)) {
     const bool is_text_box_empty =
         CreateVisibleSelection(SelectionInFlatTree::Builder()
                                    .SelectAllChildren(*inner_node)
                                    .Build())
             .IsCaret();
@@ skipping 880 matching lines @@
 
 bool IsExtendingSelection(const MouseEventWithHitTestResults& event) {
   bool is_mouse_down_on_link_or_image =
       event.IsOverLink() || event.GetHitTestResult().GetImage();
   return (event.Event().GetModifiers() & WebInputEvent::Modifiers::kShiftKey) !=
              0 &&
          !is_mouse_down_on_link_or_image;
 }
 
 }  // namespace blink