# popover_within.ludic - what an anchored popover is kept inside: the element named by within="id",
# else the nearest scroll box (or overflow box) around it, else the screen; it flips against the same
function pp_bounds(ui_st: mut UiState, p: UiNode) -> void {
  ui_st.pp_bx = ui_st.ly_vp_x
  ui_st.pp_by = ui_st.ly_vp_y
  ui_st.pp_bw = ui_st.ly_vp_w
  ui_st.pp_bh = ui_st.ly_vp_h
  var box: UiNode = null
  let id = ui_attr_text(p, "within")
  if id != "" {
    var at = p.parent
    while at != null and box == null {
      box = pp_by_id(at, id, p)
      at = at.parent
    }
    if box == null { ui_err(ui_st, `within="{id}": no element has that id`) }
  } else {
    var at = p.parent
    while at != null and box == null {
      if at.kind == UI_SCROLL { box = at }
      at = at.parent
    }
  }
  if box == null { return }
  ui_st.pp_bx = box.x
  ui_st.pp_by = box.y
  ui_st.pp_bw = box.cw
  ui_st.pp_bh = box.ch
}
# where a popover of `size` starts along its anchor's side: level with its start, centred, or its end
function pp_along(align: string, at: float, span: float, size: float) -> float {
  if align == "center" { return at + (span - size) / 2.0 }
  if align == "end" { return at + span - size }
  return at
}