code: func (win *Window) Progress(cur *int, maxval int, is_modifiable bool) bool {
  	style := &win.ctx.Style
  	state, bounds, _ := win.widget()
  	if !state {
  		return false
  	}
  
  	in := win.inputMaybe(state)
  	old_value := *cur
  	*cur = doProgress(win, bounds, *cur, maxval, is_modifiable, &style.Progress, in)
  	return *cur != old_value
  }
docs: // Adds a progress bar to win. if is_modifiable is true the progress
  // bar will be user modifiable through click-and-drag.
  // Returns true when the progress bar values is modified.