[WIP: not for review] Reduce re-layout

ui/gfx/render_text_harfbuzz.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gfx/render_text_harfbuzz.h"
 
 #include <limits>
 #include <set>
 
 #include "base/i18n/bidi_line_iterator.h"
@@ skipping 764 matching lines @@
 std::unique_ptr<RenderText> RenderTextHarfBuzz::CreateInstanceOfSameType()
     const {
   return base::WrapUnique(new RenderTextHarfBuzz);
 }
 
 bool RenderTextHarfBuzz::MultilineSupported() const {
   return true;
 }
 
 const base::string16& RenderTextHarfBuzz::GetDisplayText() {
-  // TODO(oshima): Consider supporting eliding multi-line text.
-  // This requires max_line support first.
-  if (multiline() ||
-      elide_behavior() == NO_ELIDE ||
-      elide_behavior() == FADE_TAIL) {
+  // TODO(krb): Consider other elision modes for multiline.
+  if ((multiline() && (max_lines() == 0 || elide_behavior() != ELIDE_TAIL)) ||
+      elide_behavior() == NO_ELIDE || elide_behavior() == FADE_TAIL) {
     // Call UpdateDisplayText to clear |display_text_| and |text_elided_|
     // on the RenderText class.
     UpdateDisplayText(0);
     update_display_text_ = false;
     display_run_list_.reset();
     return layout_text();
   }
 
   EnsureLayoutRunList();
   DCHECK(!update_display_text_);
@@ skipping 764 matching lines @@
     UpdateDisplayText(multiline() ? 0 : layout_run_list_.width());
     update_display_text_ = false;
     update_display_run_list_ = text_elided();
   }
 }
 
 base::i18n::BreakIterator* RenderTextHarfBuzz::GetGraphemeIterator() {
   if (update_grapheme_iterator_) {
     update_grapheme_iterator_ = false;
     grapheme_iterator_.reset(new base::i18n::BreakIterator(
         GetDisplayText(),

[tapted, 2016/05/11 00:16:26]

So thinking about it some more - I think this is the biggest culprit.

This gets called from ApplyStyle via IsValidCursorIndex(..) (_before_ the style
is applied even).

There's a comment "Do not change styles mid-grapheme to avoid breaking
ligatures", so I guess we can't just switch to using IsValidLogicalIndex in
ApplyStyle(). But does it make sense to validate the range of the style being
applied based on the _elided_ text?

Should there be something like IsValidLayoutIndex(..) which validates the style
range based on the layout_text() instead of the display_text()? This is all
needed _before_ the style is applied so the layout_text() could immediately
change anyway, meaning a previously valid index is now elided once the style is
applied.

Of course, we'd also need an updated `IndexOfAdjacentGrapheme(..)` to support
this use-case as well.

         base::i18n::BreakIterator::BREAK_CHARACTER));
     if (!grapheme_iterator_->Init())
       grapheme_iterator_.reset();
   }
   return grapheme_iterator_.get();
 }
 
 internal::TextRunList* RenderTextHarfBuzz::GetRunList() {
   DCHECK(!update_layout_run_list_);
   DCHECK(!update_display_run_list_);
   return text_elided() ? display_run_list_.get() : &layout_run_list_;
 }
 
 const internal::TextRunList* RenderTextHarfBuzz::GetRunList() const {
   return const_cast<RenderTextHarfBuzz*>(this)->GetRunList();
 }
 
 }  // namespace gfx