Visual Studio 2019 (version 16.0) introduced the ability to double click on SDK-style projects in Solution Explorer and having them open in the editor.
Based on feedback, an option has been added in Visual Studio 16.1 (currently in preview) that restores the previous behaviour.
If you prefer the old behaviour, uncheck this setting.
EDITORIAL: SDK-style projects are very readable and easy to edit by hand. If you use SDK projects you may start to think of project files as regular source file and that double clicking to edit makes sense. Of course you may not, but it’s worth considering.