This is the dummy.htm document.
It is the default document that will always be loaded in the display window if no other document is specified.
If it is not loaded within another window it will reload the test window and set the contents of the display frame to dummy.htm.
Alternatively if it is in another window but that window is not the test window it will reload the test window.
Note, if you are trying this offline in Internet Explorer, then the process will not work correctly.
Why not attempt to load display.htm as the parent window.
Load display.htm in the display window.