Revert "Make TabLayer recycling sticky to tab_id."

chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.cc

Index: chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.cc
diff --git a/chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.cc b/chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.cc
index c61ff1f1389ae84ad1859eea62344e95c97a7034..a942971bd876fa20a420a2f3c64eda3d7953aa22 100644
--- a/chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.cc
+++ b/chrome/browser/android/compositor/scene_layer/tab_list_scene_layer.cc
@@ -19,6 +19,7 @@ namespace android {
 TabListSceneLayer::TabListSceneLayer(JNIEnv* env, jobject jobj)
     : SceneLayer(env, jobj),
       content_obscures_self_(false),
+      write_index_(0),
       resource_manager_(nullptr),
       layer_title_cache_(nullptr),
       tab_content_manager_(nullptr),
@@ -32,25 +33,14 @@ TabListSceneLayer::~TabListSceneLayer() {
 
 void TabListSceneLayer::BeginBuildingFrame(JNIEnv* env,
                                            const JavaParamRef<jobject>& jobj) {
+  write_index_ = 0;
   content_obscures_self_ = false;
-
-  // Remove (and re-add) all layers every frame to guarantee that z-order
-  // matches PutTabLayer call order.
-  for (auto tab : tab_map_)
-    tab.second->layer()->RemoveFromParent();
 }
 
 void TabListSceneLayer::FinishBuildingFrame(JNIEnv* env,
                                             const JavaParamRef<jobject>& jobj) {
-  // Destroy all tabs that weren't used this frame.
-  for (auto it = tab_map_.cbegin(); it != tab_map_.cend();) {
-    if (visible_tabs_this_frame_.find(it->first) ==
-        visible_tabs_this_frame_.end())
-      it = tab_map_.erase(it);
-    else
-      ++it;
-  }
-  visible_tabs_this_frame_.clear();
+  if (layers_.size() > write_index_)
+    RemoveTabLayersInRange(write_index_, layers_.size());
 }
 
 void TabListSceneLayer::UpdateLayer(
@@ -135,18 +125,8 @@ void TabListSceneLayer::PutTabLayer(
     jfloat side_border_scale,
     jboolean attach_content,
     jboolean inset_border) {
-  scoped_refptr<TabLayer> layer;
-  auto iter = tab_map_.find(id);
-  if (iter != tab_map_.end()) {
-    layer = iter->second;
-  } else {
-    layer = TabLayer::Create(incognito, resource_manager_, layer_title_cache_,
-                             tab_content_manager_);
-    tab_map_.insert(TabMap::value_type(id, layer));
-  }
-  own_tree_->AddChild(layer->layer());
-  visible_tabs_this_frame_.insert(id);
-
+  scoped_refptr<TabLayer> layer = GetNextLayer(incognito);
+  // https://crbug.com/517314: GetNextLayer() returns null in some corner cases.
   DCHECK(layer);
   if (layer) {
     layer->SetProperties(
@@ -180,9 +160,7 @@ base::android::ScopedJavaLocalRef<jobject> TabListSceneLayer::GetJavaObject(
 
 void TabListSceneLayer::OnDetach() {
   SceneLayer::OnDetach();
-  for (auto tab : tab_map_)
-    tab.second->layer()->RemoveFromParent();
-  tab_map_.clear();
+  RemoveAllRemainingTabLayers();
 }
 
 bool TabListSceneLayer::ShouldShowBackground() {
@@ -193,6 +171,40 @@ SkColor TabListSceneLayer::GetBackgroundColor() {
   return background_color_;
 }
 
+void TabListSceneLayer::RemoveAllRemainingTabLayers() {
+  if (layers_.size() > 0)
+    RemoveTabLayersInRange(0, layers_.size());
+}
+
+void TabListSceneLayer::RemoveTabLayersInRange(unsigned start, unsigned end) {
+  DCHECK_LT(start, end);
+  DCHECK_LE(end, layers_.size());
+  DCHECK_LE(0u, start);
+  for (unsigned i = start; i < end; ++i)
+    layers_[i]->layer()->RemoveFromParent();
+  layers_.erase(layers_.begin() + start, layers_.begin() + end);
+}
+
+scoped_refptr<TabLayer> TabListSceneLayer::GetNextLayer(bool incognito) {
+  while (write_index_ < layers_.size()) {
+    scoped_refptr<TabLayer> potential = layers_[write_index_];
+    if (potential->is_incognito() == incognito)
+      break;
+    potential->layer()->RemoveFromParent();
+    layers_.erase(layers_.begin() + write_index_);
+  }
+
+  if (write_index_ < layers_.size())
+    return layers_[write_index_++];
+
+  scoped_refptr<TabLayer> layer = TabLayer::Create(
+      incognito, resource_manager_, layer_title_cache_, tab_content_manager_);
+  layers_.push_back(layer);
+  own_tree_->AddChild(layer->layer());
+  write_index_++;
+  return layer;
+}
+
 static jlong Init(JNIEnv* env, const JavaParamRef<jobject>& jobj) {
   // This will automatically bind to the Java object and pass ownership there.
   TabListSceneLayer* scene_layer = new TabListSceneLayer(env, jobj);