This is a super dumb port of [this gist](https://gist.github.com/vkravets/5370589). It should be functionally equivalent. I ported the code because I had trouble getting it to work with jruby. This is confirmed to work on Fedora 23 with jruby 9.0.4.0. All credits to the original author Vladimir Kravets.

X11FullscreenHelper.rb

# This is a port of [this gist](https://gist.github.com/vkravets/5370589)
# I literally copied the code and ported it line by line, so all credits belong to the author, Vladimir Kravets.

require 'java'

# I guess this is necessary..
# I'm on Fedora 23, I had to install the packages jna and jna-contrib
# on other platforms these two jar files might live in other locations
require '/usr/share/java/jna.jar'
require '/usr/share/java/jna/jna-platform.jar'

java_import 'com.sun.jna.Native'
java_import 'com.sun.jna.NativeLong'
java_import 'com.sun.jna.platform.unix.X11'

class X11FullscreenHelper
  NET_WM_STATE_REMOVE = 0
  NET_WM_STATE_ADD    = 1

  attr_reader :isFullScreenMode

  def initialize
    @isFullScreenMode = false
  end

  def setFullScreenWindow(w, fullScreen)
    x = X11.INSTANCE
    display = nil
    begin
      display = x.XOpenDisplay(nil)

      result = sendClientMessage(
        display,
        Native.getWindowID(w),
        "_NET_WM_STATE",
        [
          NativeLong.new(fullScreen ? NET_WM_STATE_ADD : NET_WM_STATE_REMOVE),
          x.XInternAtom(display, "_NET_WM_STATE_FULLSCREEN", false),
          x.XInternAtom(display, "_NET_WM_STATE_ABOVE", false),
          NativeLong.new(0),
          NativeLong.new(0)
        ].to_java(NativeLong)
      )
      @isFullScreenMode = (result != 0) && fullScreen;
      result != 0
    ensure
      x.XCloseDisplay(display) if display
    end
  end

  def sendClientMessage(display, wid, msg, data)
    raise ArgumentException unless data.length == 5
    x = X11.INSTANCE
    event = X11::XEvent.new
    event.type = X11::ClientMessage
    event.setType(X11::XClientMessageEvent.java_class)
    event.xclient.type = X11::ClientMessage
    event.xclient.serial = NativeLong.new(0)
    event.xclient.send_event = 1
    event.xclient.message_type = x.XInternAtom(display, msg, false)
    event.xclient.window = X11::Window.new(wid)
    event.xclient.format = 32
    event.xclient.data.setType([].to_java(NativeLong).java_class)
    event.xclient.data.l = data

    mask = NativeLong.new(X11.SubstructureRedirectMask | X11.SubstructureNotifyMask)
    result = x.XSendEvent(display, x.XDefaultRootWindow(display), 0, mask, event)
    x.XFlush(display)
    result
  end
end