Implement desktop compatibility viewport

cc/layer_tree_impl.cc

Index: cc/layer_tree_impl.cc
diff --git a/cc/layer_tree_impl.cc b/cc/layer_tree_impl.cc
index 6b04f7a872644397958872894809f11af8171c91..eeff39a96233ba9a717b2e44d06c5d0b7467d678 100644
--- a/cc/layer_tree_impl.cc
+++ b/cc/layer_tree_impl.cc
@@ -120,6 +120,38 @@ void LayerTreeImpl::UpdateMaxScrollOffset() {
   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())