Reduce canonicalization when dragging

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 369 matching lines @@
   // |dispatchSelectStart()| can change them by "selectstart" event handler.
 
   PositionInFlatTree basePosition;
   if (m_selectionState != SelectionState::ExtendedSelection) {
     // Always extend selection here because it's caused by a mouse drag
     m_selectionState = SelectionState::ExtendedSelection;
     basePosition = targetPosition.deepEquivalent();
   } else {
     basePosition = selection().computeVisibleSelectionInFlatTree().base();
   }
+  if (basePosition.isNull())

[yosin_UTC9, 2017/03/16 02:30:05]

Could you move this change in another patch?
with adding |DCHECK(basePosition.isNotNull())| in |applySelectAll()|.

[yoichio, 2017/03/16 04:10:59]

This needs because of SelectionEditor change.
Somehow in L346-350, we get "valid" |targetPosition| but null 
 |basePosition|.
I'm suspicious about this code that we get VSinDOM first and 
 covert it to VSinFlat. We should get VSinFlat directly?
Anyway this should be done in another patch.

+    return;
+
   const SelectionInFlatTree& appliedSelection = applySelectAll(
       basePosition, targetPosition.deepEquivalent(), mousePressNode,
       dragStartPos, target, hitTestResult.localPoint());
   SelectionInFlatTree::Builder builder(appliedSelection);
 
   if (selection().granularity() != CharacterGranularity)
     builder.setGranularity(selection().granularity());
 
   setNonDirectionalSelectionIfNeeded(builder.build(), selection().granularity(),
                                      AdjustEndpointsAtBidiBoundary,
@@ skipping 688 matching lines @@
 
 bool isExtendingSelection(const MouseEventWithHitTestResults& event) {
   bool isMouseDownOnLinkOrImage =
       event.isOverLink() || event.hitTestResult().image();
   return (event.event().modifiers() & WebInputEvent::Modifiers::ShiftKey) !=
              0 &&
          !isMouseDownOnLinkOrImage;
 }
 
 }  // namespace blink