Form.TopMost makes the Form a system modal window. It's supposed to be ontop of all other windows. If you only want it to be a top-level form for your application, then either set the Owner property of the child forms to the main form of your application, or call AddOwnedForm on the main form, passing the child forms as parameters. See the documentation for the Owner property and AddOwnedForm method in the Form documentation in the .NET Framework SDK for more information.
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