5 files changed, 39 insertions, 3 deletions

diff --git a/cc/layer_tree_host.cc b/cc/layer_tree_host.cc
index 8ce6a29..cddb09b 100644
--- a/cc/layer_tree_host.cc
+++ b/cc/layer_tree_host.cc
@@ -328,6 +328,7 @@ void LayerTreeHost::finishCommitOnImplThread(LayerTreeHostImpl* hostImpl)
         if (syncTree->ContentsTexturesPurged())
             syncTree->ResetContentsTexturesPurged();
     }
+    syncTree->ResetViewportSizeInvalid();
 
     m_commitNumber++;
 }
diff --git a/cc/layer_tree_host_impl.cc b/cc/layer_tree_host_impl.cc
index e6c0de7..397a2d3 100644
--- a/cc/layer_tree_host_impl.cc
+++ b/cc/layer_tree_host_impl.cc
@@ -234,6 +234,10 @@ bool LayerTreeHostImpl::canDraw()
         TRACE_EVENT_INSTANT0("cc", "LayerTreeHostImpl::canDraw empty viewport");
         return false;
     }
+    if (m_activeTree->ViewportSizeInvalid()) {
+        TRACE_EVENT_INSTANT0("cc", "LayerTreeHostImpl::canDraw viewport size recently changed");
+        return false;
+    }
     if (!m_renderer) {
         TRACE_EVENT_INSTANT0("cc", "LayerTreeHostImpl::canDraw no renderer");
         return false;
@@ -1102,6 +1106,9 @@ void LayerTreeHostImpl::setViewportSize(const gfx::Size& layoutViewportSize, con
     if (layoutViewportSize == m_layoutViewportSize && deviceViewportSize == m_deviceViewportSize)
         return;
 
+    if (pendingTree() && m_deviceViewportSize != deviceViewportSize)
+        activeTree()->SetViewportSizeInvalid();
+
     m_layoutViewportSize = layoutViewportSize;
     m_deviceViewportSize = deviceViewportSize;
 
diff --git a/cc/layer_tree_impl.cc b/cc/layer_tree_impl.cc
index 832ecaa..b0d071e 100644
--- a/cc/layer_tree_impl.cc
+++ b/cc/layer_tree_impl.cc
@@ -27,6 +27,7 @@ LayerTreeImpl::LayerTreeImpl(LayerTreeHostImpl* layer_tree_host_impl)
       max_page_scale_factor_(0),
       scrolling_layer_id_from_previous_tree_(0),
       contents_textures_purged_(false),
+      viewport_size_invalid_(false),
       needs_update_draw_properties_(true),
       needs_full_tree_sync_(true) {
 }
@@ -104,6 +105,11 @@ void LayerTreeImpl::pushPropertiesTo(LayerTreeImpl* target_tree) {
   else
     target_tree->ResetContentsTexturesPurged();
 
+  if (ViewportSizeInvalid())
+    target_tree->SetViewportSizeInvalid();
+  else
+    target_tree->ResetViewportSizeInvalid();
+
   if (hud_layer())
     target_tree->set_hud_layer(static_cast<HeadsUpDisplayLayerImpl*>(
         LayerTreeHostCommon::findLayerInSubtree(
@@ -346,6 +352,20 @@ void LayerTreeImpl::ResetContentsTexturesPurged() {
   layer_tree_host_impl_->OnCanDrawStateChangedForTree(this);
 }
 
+bool LayerTreeImpl::ViewportSizeInvalid() const {
+  return viewport_size_invalid_;
+}
+
+void LayerTreeImpl::SetViewportSizeInvalid() {
+  viewport_size_invalid_ = true;
+  layer_tree_host_impl_->OnCanDrawStateChangedForTree(this);
+}
+
+void LayerTreeImpl::ResetViewportSizeInvalid() {
+  viewport_size_invalid_ = false;
+  layer_tree_host_impl_->OnCanDrawStateChangedForTree(this);
+}
+
 Proxy* LayerTreeImpl::proxy() const {
   return layer_tree_host_impl_->proxy();
 }
diff --git a/cc/layer_tree_impl.h b/cc/layer_tree_impl.h
index 092027e..729c3d45 100644
--- a/cc/layer_tree_impl.h
+++ b/cc/layer_tree_impl.h
@@ -179,6 +179,12 @@ class CC_EXPORT LayerTreeImpl {
   void SetContentsTexturesPurged();
   void ResetContentsTexturesPurged();
 
+  // Set on the active tree when the viewport size recently changed
+  // and the active tree's size is now out of date.
+  bool ViewportSizeInvalid() const;
+  void SetViewportSizeInvalid();
+  void ResetViewportSizeInvalid();
+
   // Useful for debug assertions, probably shouldn't be used for anything else.
   Proxy* proxy() const;
 
@@ -211,6 +217,7 @@ protected:
   LayerList render_surface_layer_list_;
 
   bool contents_textures_purged_;
+  bool viewport_size_invalid_;
   bool needs_update_draw_properties_;
 
   // In impl-side painting mode, this is true when the tree may contain
diff --git a/cc/thread_proxy.cc b/cc/thread_proxy.cc
index 9e7773c..ddc61c0 100644
--- a/cc/thread_proxy.cc
+++ b/cc/thread_proxy.cc
@@ -1176,9 +1176,10 @@ void ThreadProxy::renewTreePriority()
     if (m_smoothnessTakesPriorityExpirationTime > base::TimeTicks::Now())
         priority = SMOOTHNESS_TAKES_PRIORITY;
 
-    // New content always takes priority when we have an active tree with
-    // evicted resources.
-    if (m_layerTreeHostImpl->activeTree()->ContentsTexturesPurged())
+    // New content always takes priority when the active tree has
+    // evicted resources or there is an invalid viewport size.
+    if (m_layerTreeHostImpl->activeTree()->ContentsTexturesPurged() ||
+        m_layerTreeHostImpl->activeTree()->ViewportSizeInvalid())
         priority = NEW_CONTENT_TAKES_PRIORITY;
 
     m_layerTreeHostImpl->setTreePriority(priority);