Misc de-jelly fixes.

Fixes two small issues:
- Padding of visible layer region did not consider page scale.
- New layers may jitter as we won't de-jelly them for 1 frame. We now
  use a reasonable default.

Bug: 995965
Change-Id: Ib2ad58eab9b393b691e586e8649672c1d4323864
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1861096Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705781}

cc/trees/de_jelly_state.cc

@@ -54,6 +54,17 @@ void DeJellyState::AdvanceFrame(LayerTreeImpl* layer_tree_impl) {
     return;
   }
 
+  // Compute the fallback movement of a scrolling element based purely on the
+  // scroll offset of the currently scrolling node.
+  float previous_scroll_offset = scroll_offset_;
+  scroll_offset_ = layer_tree_impl->property_trees()
+                       ->transform_tree.Node(scroll_transform_node_)
+                       ->scroll_offset.y();
+  fallback_delta_y_ = scroll_offset_ - previous_scroll_offset;
+  gfx::Vector3dF vector(0, fallback_delta_y_, 0);
+  new_scroll_node_transform_->TransformVector(&vector);
+  fallback_delta_y_ = vector.y();
+
   // Don't attempt de-jelly while the omnibox is transitioning in or out. There
   // is no correct way to handle this.
   float top_controls_shown_ratio =
@@ -115,17 +126,20 @@ void DeJellyState::UpdateSharedQuadState(LayerTreeImpl* layer_tree_impl,
 
   // Get the previous transform (if any).
   const auto& found = previous_transforms_.find(transform_id);
+
+  float delta_y = 0.0f;
   if (found == previous_transforms_.end()) {
-    return;
+    delta_y = fallback_delta_y_;
+  } else {
+    // Calculate the delta of point (0, 0) from the previous frame.
+    gfx::Transform previous_transform = found->second;
+    gfx::PointF new_point(0, 0);
+    transform.TransformPoint(&new_point);
+    gfx::PointF old_point(0, 0);
+    previous_transform.TransformPoint(&old_point);
+    delta_y = old_point.y() - new_point.y();
   }
 
-  // Calculate the delta of point (0, 0) from the previous frame.
-  gfx::Transform previous_transform = found->second;
-  gfx::PointF new_point(0, 0);
-  transform.TransformPoint(&new_point);
-  gfx::PointF old_point(0, 0);
-  previous_transform.TransformPoint(&old_point);
-  float delta_y = old_point.y() - new_point.y();
   if (delta_y == 0.0f) {
     return;
   }

cc/trees/de_jelly_state.h

@@ -41,6 +41,9 @@ class CC_EXPORT DeJellyState {
  private:
   bool should_de_jelly_ = false;
   int scroll_transform_node_ = 0;
+  float scroll_offset_ = 0;
+  float fallback_delta_y_ = 0;
+
   base::Optional<gfx::Transform> new_scroll_node_transform_;
   std::map<int, gfx::Transform> previous_transforms_;
   std::map<int, gfx::Transform> new_transforms_;

cc/trees/draw_property_utils.cc

@@ -642,7 +642,11 @@ gfx::Rect LayerVisibleRect(PropertyTrees* property_trees, LayerImpl* layer) {
 
   gfx::Rect visible_rect = ToEnclosingClipRect(clip_in_layer_space);
   if (layer->layer_tree_impl()->settings().allow_de_jelly_effect) {
-    visible_rect.Inset(0.0f, -viz::MaxDeJellyHeight());
+    float padding_amount = viz::MaxDeJellyHeight();
+    if (layer->IsAffectedByPageScale()) {
+      padding_amount /= layer->layer_tree_impl()->current_page_scale_factor();
+    }
+    visible_rect.Inset(0.0f, -padding_amount);
   }
   visible_rect.Intersect(layer_content_rect);
   return visible_rect;