The former fails; the latter works

gosu_char_width_erratic.rb

# encoding: ascii-8bit

# To put ö directly into a string, I need to set the encoding to ascii-8bit.

require 'gosu'
include Gosu

class Game < Window
  def initialize
    super 640, 480, false
  end
  
  def update
    f = Font.new(self, nil, 16)

    p f.text_width("o")
    p f.text_width("ö")

    self.text_input = TextInput.new
    text_input.text += "ö"
    p f.text_width(text_input.text)
  
    close
  end
end

Game.new.show

# Win7 x64
# 8.5
# 10.0
# 10.0

gosu_char_width_working.rb

# If I take the input from the text-input, it works fine.

require 'gosu'
include Gosu

class Game < Window
  def initialize
    super 640, 480, false
	
    @font = Font.new(self, nil, 16)
    self.text_input = TextInput.new
  end
  
  def update
    @width = @font.text_width(text_input.text)
  end
  
  def draw
    @font.draw text_input.text, 0, 0, 0
    @font.draw @width.to_s, 0, 50, 0
  end
end

Game.new.show

# This gives the width of "ö" or "o" as 8.5