Keep the selection of the text field when changed by JS.

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:
  * 1. Redistributions of source code must retain the above copyright
@@ skipping 175 matching lines @@
                     newSelection = VisibleSelection(start, pos);
             }
         } else {
             newSelection.setExtent(pos);
         }
 
         if (selection().granularity() != CharacterGranularity) {
             granularity = selection().granularity();
             expandSelectionUsingGranularity(newSelection, selection().granularity());
         }
-    } else {
+    } else if (m_selectionState != SelectionState::ExtendedSelection) {
         newSelection = expandSelectionToRespectUserSelectAll(innerNode, VisibleSelection(visiblePos));
     }
 
     // Updating the selection is considered side-effect of the event and so it doesn't impact the handled state.
     updateSelectionForMouseDownDispatchingSelectStart(innerNode, newSelection, granularity);
     return false;
 }
 
 template <typename Strategy>
 void SelectionController::updateSelectionForMouseDragAlgorithm(const HitTestResult& hitTestResult, Node* mousePressNode, const LayoutPoint& dragStartPos, const IntPoint& lastKnownMousePosition)
@@ skipping 404 matching lines @@
 bool SelectionController::mouseDownMayStartSelect() const
 {
     return m_mouseDownMayStartSelect;
 }
 
 bool SelectionController::mouseDownWasSingleClickInSelection() const
 {
     return m_mouseDownWasSingleClickInSelection;
 }
 
+void SelectionController::notifySelectionChanged()
+{
+    if (selection().selectionType() == SelectionType::RangeSelection)
+        m_selectionState = SelectionState::ExtendedSelection;
+    else if (selection().selectionType() == SelectionType::CaretSelection)
+        m_selectionState = SelectionState::PlacedCaret;
+    else
+        m_selectionState = SelectionState::HaveNotStartedSelection;
+}
+
 FrameSelection& SelectionController::selection() const
 {
     return m_frame->selection();
 }
 
 } // namespace blink