code: func Flex(nextSlide func()) (title string, content tview.Primitive) {
  	modalShown := false
  	pages := tview.NewPages()
  	textView := tview.NewTextView().
  		SetDoneFunc(func(key tcell.Key) {
  			if modalShown {
  				nextSlide()
  				modalShown = false
  			} else {
  				pages.ShowPage("modal")
  				modalShown = true
  			}
  		})
  	textView.SetBorder(true).SetTitle("Flexible width, twice of middle column")
  	flex := tview.NewFlex().
  		AddItem(textView, 0, 2, true).
  		AddItem(tview.NewFlex().
  			SetDirection(tview.FlexRow).
  			AddItem(tview.NewBox().SetBorder(true).SetTitle("Flexible width"), 0, 1, false).
  			AddItem(tview.NewBox().SetBorder(true).SetTitle("Fixed height"), 15, 1, false).
  			AddItem(tview.NewBox().SetBorder(true).SetTitle("Flexible height"), 0, 1, false), 0, 1, false).
  		AddItem(tview.NewBox().SetBorder(true).SetTitle("Fixed width"), 30, 1, false)
  	modal := tview.NewModal().
  		SetText("Resize the window to see the effect of the flexbox parameters").
  		AddButtons([]string{"Ok"}).SetDoneFunc(func(buttonIndex int, buttonLabel string) {
  		pages.HidePage("modal")
  	})
  	pages.AddPage("flex", flex, true, true).
  		AddPage("modal", modal, false, false)
  	return "Flex", pages
  }
docs: // Flex demonstrates flexbox layout.