Class Hierarchy

Overview

DOM elements & Renderers

• Document elements create RenderView.
• HTMLHtmlElement, HTMLBodyElement, HTMLDivElement etc elements create RenderBlockFlow (for the default CSS display value).
• HTMLIFrameElement elements create RenderIFrame.

Scrollable Areas

• A MainFrame is created for each document.
• A Frame is created for each HTMLIFrameElement.
• A RenderLayer is created for each RenderLayerModelObject when RenderLayerModelObject::requiresLayer() returns true.

Scrolling Nodes

A RenderLayer creates a RenderLayerBacking if RenderLayerCompositor::needsToBeComposited(layer) returns true. This relies on RenderLayerCompositor::requiresCompositingLayer and (for iframe) on RenderLayerCompositor::requiresCompositingForFrame.

The following backtrace is executed to make a layer scrollable:

      RenderLayerCompositor::updateScrollCoordinatedLayer(RenderLayer& layer, LayerScrollCoordinationRoles reasons)
      RenderLayerCompositor::updateScrollCoordinatedStatus(RenderLayer& layer)
      RenderLayerCompositor::didAddScrollingLayer(RenderLayer& layer)
      RenderLayerBacking::updateScrollingLayers(bool needsScrollingLayers)
      RenderLayerBacking::updateConfiguration()
    

The conditions for requiring a scrolling layer is:

• On iOS, RenderLayer::hasTouchScrollOverflow().
• Otherwise, RenderLayer::needsCompositedScrolling() which is updated in RenderLayer::updateNeedsCompositedScrolling(). It is always false if associated FrameView does not contain any ScrollableArea child.

The reasons to update the scroll coordinated layer include:

• ViewportConstrained:
  • "display: sticky" layers creates StickyNode.
  • "display: fixed" layers creates FixedNode.
• Scrolling:
  • The root layer (owning the RenderView) creates a FrameScrollingNode.
  • Other layers creates OverflowScrollingNode.