I was testing the prototype-ui rc0 window code with both IE6 and FireFox 2 and ran into a JavaScript error with the functional test_button.html and IE6. The error was with the line (return this.options.theme || this.windowManager.getTheme();) in the getTheme function of UI.Window. The this.windowManager is null with IE6, but is an Object with FireFox. I found that UI.defaultWM is not being defined by IE6 with the document.whenReady function in time for the initialize function of UI.Window to use. I fixed the problem with IE6 by adding some code to the initialize function of UI.Window as follows:

initialize: function(options) {
    if (Object.isUndefined(UI.defaultWM) || (UI.defaultWM == null))
    UI.defaultWM = new UI.WindowManager();
  this.setOptions(options);

The two lines starting with the if test are the ones that I added to make sure that UI.defaultWM is defined to a UI.WindowManager Object.