code: func (win *Window) TreePop() {
  	layout := win.layout
  	panel_padding := win.style().Padding
  	layout.AtX -= panel_padding.X + win.ctx.Style.Tab.Indent
  	layout.Width += panel_padding.X + win.ctx.Style.Tab.Indent
  	if layout.Row.TreeDepth == 0 {
  		panic("TreePop called without opened tree nodes")
  	}
  	win.curNode = win.curNode.Parent
  	layout.Row.TreeDepth--
  }
docs: // TreePop signals that the program is done adding elements to the
  // current collapsable section.