I had the same problems in 8.5, and I double checked the Xtra version, and it is the MX xtra. Here's the code I'm using to build the dialog box. It works fine on Windows:
In your windowPropList you don't specify anything for the modality of the dialog.
In my experience, run() expects the window.modal = 1; whereas show() expects the window.modal = 0 - I'm not sure what the default is, but it's possibly 0.
Try explicitly setting window.modal = 1, especially if you're showing it with a run() command.
HTH,
-Sean.
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