main thread regions: 1
over iframe: blocked-by-main-thread-region
outside iframe: accepted
display list:
AccumulatedVisualContext Tree:
  [1] scroll_frame_id=1 (PaintableWithLines(BlockContainer<PRE>#out))

DisplayList:
SaveLayer@0
  CompositorWheelHitTestTarget@0 target_scroll_frame_index=1 rect=[0,0 800x600]
  CompositorScrollNode@0 scroll_frame_index=1 parent_scroll_frame_index=0 scrollport_rect=[0,0 800x600] max_scroll_offset=[0,1413] is_viewport=true
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=1 rect=[0,0 800x2013]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=1 rect=[0,0 800x2000]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=1 rect=[0,0 800x2000]
  CompositorWheelHitTestTarget@1 target_scroll_frame_index=1 rect=[40,40 160x120]
  CompositorMainThreadWheelEventRegion@1 rect=[40,40 160x120]
  Save@1
    AddClipRect@1 rect=[40,40 160x120]
    DrawExternalContent@1 dst_rect=[40,40 160x120]
  Restore@1
Restore@0