code: func (mw *masterWindowCommon) PopupOpen(title string, flags WindowFlags, rect rect.Rect, scale bool, updateFn UpdateFn) {
  	go func() {
  		mw.ctx.mw.Lock()
  		defer mw.ctx.mw.Unlock()
  		mw.ctx.popupOpen(title, flags, rect, scale, updateFn)
  		mw.ctx.mw.Changed()
  	}()
  }
docs: // Opens a popup window inside win. Will return true until the
  // popup window is closed.
  // The contents of the popup window will be updated by updateFn