Support bottom-right anchored fixed-position elements during a pinch gesture

cc/layer_tree_impl.cc

 // Copyright 2011 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 "cc/layer_tree_impl.h"
 
 #include "base/debug/trace_event.h"
 #include "cc/heads_up_display_layer_impl.h"
 #include "cc/layer_tree_host_common.h"
 #include "cc/layer_tree_host_impl.h"
@@ skipping 152 matching lines @@
     if (pending_tree) {
       DCHECK_EQ(1, pending_tree->sent_page_scale_delta());
       pending_tree->SetPageScaleDelta(page_scale_delta_ / sent_page_scale_delta_);
     }
   }
 
   UpdateMaxScrollOffset();
   set_needs_update_draw_properties();
 }
 
-gfx::SizeF LayerTreeImpl::ScrollableViewportSize() const {
-  gfx::SizeF view_bounds;
+gfx::SizeF LayerTreeImpl::UnscaledScrollableViewportSize() const {
   // The clip layer should be used for scrolling bounds if available since it
   // adjusts for non-overlay scrollbars. Otherwise, fall back to our knowledge
   // of the physical viewport size.
   LayerImpl* clip_layer = NULL;
   if (root_scroll_layer_)
     clip_layer = root_scroll_layer_->parent();
   if (clip_layer && clip_layer->masksToBounds()) {
-    view_bounds = clip_layer->bounds();
+    return clip_layer->bounds();
   } else {
-    view_bounds = gfx::ScaleSize(device_viewport_size(),
+    return gfx::ScaleSize(device_viewport_size(),
         1 / device_scale_factor());
   }
+}
+
+gfx::SizeF LayerTreeImpl::ScrollableViewportSize() const {
+  gfx::SizeF view_bounds = UnscaledScrollableViewportSize();
   view_bounds.Scale(1 / total_page_scale_factor());
 
   return view_bounds;
 }
 
 void LayerTreeImpl::UpdateMaxScrollOffset() {
   if (!root_scroll_layer_ || !root_scroll_layer_->children().size())
     return;
 
   gfx::Vector2dF max_scroll = gfx::Rect(ScrollableSize()).bottom_right() -
       gfx::RectF(ScrollableViewportSize()).bottom_right();
 
   // The viewport may be larger than the contents in some cases, such as
   // having a vertical scrollbar but no horizontal overflow.
   max_scroll.ClampToMin(gfx::Vector2dF());
 
   root_scroll_layer_->setMaxScrollOffset(gfx::ToFlooredVector2d(max_scroll));
+
+  gfx::Vector2dF scrollable_viewport_size =
+      gfx::RectF(ScrollableViewportSize()).bottom_right() - gfx::PointF();
+  gfx::Vector2dF original_scrollable_viewport_size =
+      gfx::RectF(UnscaledScrollableViewportSize()).bottom_right() -
+      gfx::PointF();
+  original_scrollable_viewport_size.Scale(1 / page_scale_factor());
+  root_scroll_layer_->setFixedContainerSizeDelta(
+      scrollable_viewport_size - original_scrollable_viewport_size);
 }
 
 gfx::Transform LayerTreeImpl::ImplTransform() const {
   gfx::Transform transform;
   transform.Scale(total_page_scale_factor(), total_page_scale_factor());
   return transform;
 }
 
 struct UpdateTilePrioritiesForLayer {
   void operator()(LayerImpl *layer) {
@@ skipping 266 matching lines @@
   for (LayerIteratorType it = LayerIteratorType::begin(
            &render_surface_layer_list_); it != end; ++it) {
     if (!it.representsItself())
       continue;
     state->Append((*it)->AsValue().release());
   }
   return state.PassAs<base::Value>();
 }
 
 } // namespace cc