Don't worry, Synapse has been develop whith all the tools provides by Visual studio in order to do a safe extension. We become the default editor for C# files but we do not change the other behavior of Visual Studio.
If the codes is visible in a text editor and not in Synapse, that means that Synapse does not support this syntax yet. You can report the problem in the following link.
No, the code remains yours as your intellectual property. Synapse is only a tool. Nonetheless you can Check the term related to Visual Studio and other dependancies (Roslyn, .Net framework...).