mywidget.rb

require 'gtk3'

class MyWidget < Gtk::Widget
  type_register

  def initialize()
    super()

    self.has_window = true
    self.redraw_on_allocate = true
  end

  def request_mode
    notice 'request_mode'

    Gtk::SizeRequestMode::HEIGHT_FOR_WIDTH
  end

  def preferred_width
    notice 'preferred_width'

    [100, 250] # minimum, natural
  end

  def preferred_height
    notice 'preferred_height'

    h = preferred_height_for_width 250
    [h, h]
  end

  def preferred_height_for_width(width)
    notice "preferred_height_for_width(#{width})"

    h = 3 * width
    [h, h]
  end
end