Implement desktop compatibility viewport

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/layer_tree_host_common.h"
 #include "cc/layer_tree_host_impl.h"
 #include "ui/gfx/vector2d_conversions.h"
@@ skipping 102 matching lines @@
   gfx::Vector2dF max_scroll = gfx::Rect(ScrollableSize()).bottom_right() -
       gfx::RectF(view_bounds).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));
 }
 
+static gfx::Vector2dF toVector2dF(const gfx::Size& size) { return gfx::Vector2dF(size.width(), size.height()); }
+static gfx::Vector2dF toVector2dF(const gfx::SizeF& size) { return gfx::Vector2dF(size.width(), size.height()); }
+
+void LayerTreeImpl::UpdateFixedPositionViewport() {
+  if (!root_scroll_layer_)
+    return;
+
+  const gfx::Vector2dF total_scroll_offset = root_scroll_layer_->scrollOffset() + root_scroll_layer_->scrollDelta();
+  const gfx::Vector2dF layout_viewport_size = toVector2dF(pinch_zoom_viewport().layout_viewport_size());
+  const gfx::Vector2d desktop_compatibility_viewport_offset = root_scroll_layer_->desktopCompatibilityViewportOffsetFromScrollPosition();
+
+  gfx::Vector2dF desktop_compatibility_viewport_offset_delta = root_scroll_layer_->desktopCompatibilityViewportOffsetFromScrollPositionDelta();
+
+  // Make sure fixed position viewport contains the visible viewport
+  {
+    const gfx::Vector2dF offset_min = toVector2dF(pinch_zoom_viewport().LayoutSpaceViewportSize()) - layout_viewport_size;
+    desktop_compatibility_viewport_offset_delta.ClampToMin(offset_min - desktop_compatibility_viewport_offset);
+    const gfx::Vector2dF offset_max;
+    desktop_compatibility_viewport_offset_delta.ClampToMax(offset_max - desktop_compatibility_viewport_offset);
+  }
+
+  // Clamp the fixed position viewport within the document bounds.
+  {
+    const gfx::Vector2dF offset_max = toVector2dF(ScrollableSize()) - layout_viewport_size - total_scroll_offset;
+    desktop_compatibility_viewport_offset_delta.ClampToMax(offset_max - desktop_compatibility_viewport_offset);
+    const gfx::Vector2dF offset_min = -total_scroll_offset;
+    desktop_compatibility_viewport_offset_delta.ClampToMin(offset_min - desktop_compatibility_viewport_offset);
+  }
+
+  root_scroll_layer_->setDesktopCompatibilityViewportOffsetFromScrollPositionDelta(desktop_compatibility_viewport_offset_delta);
+}
+
 void LayerTreeImpl::UpdateDrawProperties() {
   render_surface_layer_list_.clear();
   if (!RootLayer())
     return;
 
   if (root_scroll_layer_) {
     root_scroll_layer_->setImplTransform(
         layer_tree_host_impl_->implTransform());
   }
 
@@ skipping 197 matching lines @@
 
 AnimationRegistrar* LayerTreeImpl::animationRegistrar() const {
   return layer_tree_host_impl_->animationRegistrar();
 }
 
 const PinchZoomViewport& LayerTreeImpl::pinch_zoom_viewport() const {
   return layer_tree_host_impl_->pinchZoomViewport();
 }
 
 } // namespace cc