Making it an option is a separate feature request.
I expressed the idea of an option just in case a developer would have said: "It is OK that the editor is not in focus when opening a document". Personally I'm not interested in an option because I don't see a need. But if someone will ask an option in the future, then maybe those thoughts could be interesting:
Add two checkbox in `Preferences`, in `Editor` in `Features`: `if sidebar has focus, the editor will be in focus when opening a document` `if message window has focus, the editor will be in focus when opening a document` Both checkbox should be checked by default.