https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.form.isrestrictedwindow?view=netframework-4.8.1&redirectedfrom=MSDN
Form.IsRestrictedWindow Property (System.Windows.Forms) | Microsoft Learn
Gets a value indicating whether the form can use all windows and user input events without restriction.
property systemwindows formsmicrosoftlearn