Update composition underline start/end constraints.

third_party/WebKit/Source/core/layout/line/InlineTextBox.h

 /*
  * (C) 1999 Lars Knoll (knoll@kde.org)
  * (C) 2000 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * 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
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 17 matching lines @@
 #include "core/layout/api/SelectionState.h"
 #include "core/layout/line/InlineBox.h"
 #include "platform/text/TextRun.h"
 #include "wtf/Forward.h"
 
 namespace blink {
 
 class DocumentMarker;
 class GraphicsContext;
 
+// The two truncation values below are used as tokens representing truncation
+// state for the text box, are intended to be relative to |m_start|, and are set
+// directly into |m_truncation|. In the case where there is some truncation of
+// the text but it is not full, |m_truncation| is set to the character offset
+// from |m_start| representing the characters that are not truncated.
+//
+// Thus the maximum possible length of the text displayed before an ellipsis in
+// a single InlineTextBox is |USHRT_MAX - 2| to allow for the no-truncation and
+// full-truncation states.
 const unsigned short cNoTruncation = USHRT_MAX;
 const unsigned short cFullTruncation = USHRT_MAX - 1;
 
 class CORE_EXPORT InlineTextBox : public InlineBox {
 public:
     InlineTextBox(LineLayoutItem item, int start, unsigned short length)
         : InlineBox(item)
         , m_prevTextBox(nullptr)
         , m_nextTextBox(nullptr)
         , m_start(start)
@@ skipping 133 matching lines @@
         return (canHaveLeadingExpansion() ? TextRun::AllowLeadingExpansion : TextRun::ForbidLeadingExpansion)
             | (expansion() && nextLeafChild() ? TextRun::AllowTrailingExpansion : TextRun::ForbidTrailingExpansion);
     }
 };
 
 DEFINE_INLINE_BOX_TYPE_CASTS(InlineTextBox);
 
 } // namespace blink
 
 #endif // InlineTextBox_h