Why doesn't a red rectangle appear within Thread block?

thread_test_jruby_swt.rb

require 'java'
require 'swt'

module Swt
  include_package 'org.eclipse.swt'
  include_package 'org.eclipse.swt.widgets'
  include_package 'org.eclipse.swt.graphics'
end

class Object
  include Swt
end

display = Swt::Display.new
shell = Swt::Shell.new display
shell.setSize 300+16, 300+38
cs = Swt::Composite.new shell, Swt::SWT::NONE
cs.setSize 300, 300

shell.open

#Thread.new do
  gc = Swt::GC.new cs
  gc.setBackground Swt::Color.new(display, 255, 0, 0)
  gc.fillRectangle 100, 100, 100, 100
#end

while !shell.isDisposed do
  display.sleep unless display.readAndDispatch
end
display.dispose

The above snippet works. A red rectangle appears on a window. But after deleting number sign #s, I mean using Thread, nothing appear. Why?