Function place_aligned 

pub fn place_aligned(
    anchor: Rect,
    size: (f64, f64),
    side: PopoverSide,
    align: PanelAlign,
    gap: f64,
    viewport: Rect,
) -> (Rect, PopoverSide)

place_anchored with the cross-axis edge it lines up with: a panel aligned PanelAlign::Trailing hangs from the anchor’s trailing edge and flips to the leading edge only when it would run past the viewport’s leading edge.