I'm using 12.1. XamDialogWindow being set to IsModal=True is not working like  a WinForm dialog window.

Quoting the online help: "Modal dialog – This dialog window always maintains the active window status until it is closed. When a modal dialog window is opened, your end user cannot interact with any other part of the application until the dialog window is closed"

Look at the sample provided with the product. Click "Open Modal Window". Dialog windows are supposed to keep focus and block access to the rest of the app until it's closed. This is not the behavior. You can click on the other tabs (xaml, snippet, etc), the toolbar type buttons on the top are accessible and the app options on the left are also accessible. Click the XAML tab, then back to the Sample tab (which should not be allowed) and the Dialog Window is closed.

Does XamDialogWindow work as a modal window?