SMenuAnchor.cpp

  FGeometry ComputeMenuPlacement(const FGeometry& AllottedGeometry, const FVector2D& PopupDesiredSize, EMenuPlacement PlacementMode)
    {
	// Compute the popup size, offset, and anchor rect  in local space
	const FPopupPlacement Placement(AllottedGeometry, PopupDesiredSize, PlacementMode);

	// ask the application to compute the proper desktop offset for the anchor. This requires the offsets to be in desktop space.
	const FVector2D NewPositionDesktopSpace = FSlateApplication::Get().CalculatePopupWindowPosition(
		TransformRect(AllottedGeometry.GetAccumulatedLayoutTransform(), FSlateRect(Placement.AnchorLocalSpace.GetTopLeft() + Placement.LocalPopupOffset, Placement.AnchorLocalSpace\
		.GetBottomRight() + Placement.LocalPopupOffset)),
		TransformVector(AllottedGeometry.GetAccumulatedLayoutTransform(), Placement.LocalPopupSize),
		Placement.Orientation);

	// transform the desktop offset into local space and use that as the layout transform for the child content.
	return AllottedGeometry.MakeChild(
		Placement.LocalPopupSize,		
		FSlateLayoutTransform(TransformPoint(Inverse(AllottedGeometry.GetAccumulatedLayoutTransform()), NewPositionDesktopSpace)));
    }