Stabilize LayoutSelection

This patch stabilizes LayoutSelection by letting it be off from
LayoutObject reference.
Design doc: https://bit.ly/2KT87L9

Algorithm updates:
 Mark SelectionState::kContain to all ascendants of each leaf selected node on flat tree.
 Do nothing while Node/LayoutObject::RemoveChild().
 At prepaint phase, collect all selected LayoutObject from root using kContain on flat tree.

Bug: 843144
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ib4b39c621043d5fe03f46e0ce8b4d2b972e9ed3e
Reviewed-on: https://chromium-review.googlesource.com/1125709Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Yoichi Osato <yoichio@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577114}

third_party/WebKit/LayoutTests/flag-specific/enable-slimming-paint-v2/paint/invalidation/selection/selection-clear-expected.txt

@@ -57,14 +57,6 @@
     {
       "object": "RootInlineBox",
       "reason": "geometry"
-    },
-    {
-      "object": "LayoutBR BR",
-      "reason": "selection"
-    },
-    {
-      "object": "InlineTextBox '\n'",
-      "reason": "selection"
     }
   ]
 }

third_party/WebKit/LayoutTests/platform/linux/paint/invalidation/selection/selection-clear-expected.txt

@@ -68,14 +68,6 @@
     {
       "object": "RootInlineBox",
       "reason": "geometry"
-    },
-    {
-      "object": "LayoutBR BR",
-      "reason": "selection"
-    },
-    {
-      "object": "InlineTextBox '\n'",
-      "reason": "selection"
     }
   ]
 }

third_party/WebKit/LayoutTests/platform/mac/paint/invalidation/selection/selection-clear-expected.txt

@@ -68,14 +68,6 @@
     {
       "object": "RootInlineBox",
       "reason": "geometry"
-    },
-    {
-      "object": "LayoutBR BR",
-      "reason": "selection"
-    },
-    {
-      "object": "InlineTextBox '\n'",
-      "reason": "selection"
     }
   ]
 }

third_party/WebKit/LayoutTests/platform/win/paint/invalidation/selection/selection-clear-expected.txt

@@ -68,14 +68,6 @@
     {
       "object": "RootInlineBox",
       "reason": "geometry"
-    },
-    {
-      "object": "LayoutBR BR",
-      "reason": "selection"
-    },
-    {
-      "object": "InlineTextBox '\n'",
-      "reason": "selection"
     }
   ]
 }

third_party/blink/renderer/core/editing/layout_selection.h

@@ -34,60 +34,6 @@ class LayoutObject;
 class NGPaintFragment;
 class FrameSelection;
 struct LayoutSelectionStatus;
-// This class represents a selection range in layout tree for painting and
-// paint invalidation.
-// The current selection to be painted is represented as 2 pairs of
-// (LayoutObject, offset).
-// 2 LayoutObjects are only valid for |Text| node without 'transform' or
-// 'first-letter'.
-// TODO(editing-dev): Clarify the meaning of "offset".
-// editing/ passes them as offsets in the DOM tree but layout uses them as
-// offset in the layout tree. This doesn't work in the cases of
-// CSS first-letter or character transform. See crbug.com/17528.
-class SelectionPaintRange {
-  DISALLOW_NEW();
-
- public:
-  class Iterator
-      : public std::iterator<std::input_iterator_tag, LayoutObject*> {
-   public:
-    explicit Iterator(const SelectionPaintRange*);
-    Iterator(const Iterator&) = default;
-    bool operator==(const Iterator& other) const {
-      return current_ == other.current_;
-    }
-    bool operator!=(const Iterator& other) const { return !operator==(other); }
-    Iterator& operator++();
-    LayoutObject* operator*() const;
-
-   private:
-    LayoutObject* current_;
-    const LayoutObject* stop_;
-  };
-  Iterator begin() const { return Iterator(this); };
-  Iterator end() const { return Iterator(nullptr); };
-
-  SelectionPaintRange() = default;
-  SelectionPaintRange(LayoutObject* start_layout_object,
-                      base::Optional<unsigned> start_offset,
-                      LayoutObject* end_layout_object,
-                      base::Optional<unsigned> end_offset);
-
-  bool operator==(const SelectionPaintRange& other) const;
-
-  LayoutObject* StartLayoutObject() const;
-  base::Optional<unsigned> StartOffset() const;
-  LayoutObject* EndLayoutObject() const;
-  base::Optional<unsigned> EndOffset() const;
-
-  bool IsNull() const { return !start_layout_object_; }
-
- private:
-  LayoutObject* start_layout_object_ = nullptr;
-  base::Optional<unsigned> start_offset_ = base::nullopt;
-  LayoutObject* end_layout_object_ = nullptr;
-  base::Optional<unsigned> end_offset_ = base::nullopt;
-};
 
 class LayoutSelection final : public GarbageCollected<LayoutSelection> {
  public:
@@ -102,6 +48,7 @@ class LayoutSelection final : public GarbageCollected<LayoutSelection> {
   IntRect AbsoluteSelectionBounds();
   void InvalidatePaintForSelection();
 
+  // TODO(yoichio): Remove this function since this doesn't do anything.
   void ClearSelection();
   base::Optional<unsigned> SelectionStart() const;
   base::Optional<unsigned> SelectionEnd() const;
@@ -116,8 +63,19 @@ class LayoutSelection final : public GarbageCollected<LayoutSelection> {
 
   Member<FrameSelection> frame_selection_;
   bool has_pending_selection_ : 1;
-
-  SelectionPaintRange paint_range_;
+  // Each offset represents text offsets on selection edge if it is text.
+  // For example, suppose we select "f^oo<br><img>|",
+  // |start_offset_| is 1 and |end_offset_| is nullopt.
+  // Each of them is only valid for a |Text| node without 'transform' or
+  // 'first-letter'.
+  // TODO(editing-dev): Clarify the meaning of "offset".
+  // editing/ passes them as offsets in the DOM tree but layout uses them as
+  // offset in the layout tree. This doesn't work in the cases of
+  // character transform. See crbug.com/17528.
+  base::Optional<unsigned> start_offset_;
+  base::Optional<unsigned> end_offset_;
+  // This is true if at least one LayoutObject has a valid SelectionState.
+  bool has_selection_;
 };
 
 void CORE_EXPORT PrintLayoutObjectForSelection(std::ostream&, LayoutObject*);

third_party/blink/renderer/core/layout/api/selection_state.h

@@ -10,25 +10,32 @@
 
 namespace blink {
 
+// Each LayoutObject has a SelectionState and it represents how the
+// LayoutObject is selected. This enum is used to paint/invalidate selection
+// highlight for the LayoutObject.
 enum class SelectionState {
-  // LayoutObject is not selected.
+  // The LayoutObject is not selected.
   kNone,
-  // LayoutObject is the start of a selection run and doesn't have children.
+  // kStart, kInside, kEnd and kStartAndEnd represent the LayoutObject
+  // is somehow selected to paint and either LayoutText or LayoutReplaced.
+  // The start of a selection.
   kStart,
-  // LayoutObject is fully encompassed by a selection run and
-  // doesn't have children.
+  // Inside a selection.
   kInside,
-  // LayoutObject is the end of a selection run and doesn't have children.
+  // The end of a selection.
   kEnd,
-  // LayoutObject contains an entire selection run and doesn't have children.
+  // The LayoutObject contains an entire selection.
   kStartAndEnd,
-  // LayoutObject has at least one LayoutObject child which SelectionState is
-  // kStart, kInside, kEnd or kStartAndEnd.
-  // This property is used to invalidate LayoutObject for ::selection style
-  // change. See LayoutObject::InvalidatePaintForSelection().
+  // The LayoutObject has at least one LayoutObject child which SelectionState
+  // is not KNone.
+  // This property is used to invalidate LayoutObject.
   kContain
 };
 
+inline bool IsSelected(const SelectionState state) {
+  return state != SelectionState::kNone && state != SelectionState::kContain;
+}
+
 CORE_EXPORT std::ostream& operator<<(std::ostream&, const SelectionState);
 
 }  // namespace blink

third_party/blink/renderer/core/paint/replaced_painter.cc

@@ -108,7 +108,7 @@ void ReplacedPainter::Paint(const PaintInfo& paint_info) {
   // want it to run right up to the edges of surrounding content.
   bool draw_selection_tint =
       local_paint_info.phase == PaintPhase::kForeground &&
-      layout_replaced_.GetSelectionState() != SelectionState::kNone &&
+      IsSelected(layout_replaced_.GetSelectionState()) &&
       !local_paint_info.IsPrinting();
   if (draw_selection_tint && !DrawingRecorder::UseCachedDrawingIfPossible(
                                  local_paint_info.context, layout_replaced_,