I just fixed it! It was disabled globally because text selection causes different problems with clickable / drawable / draggable parts of the UI. microStudio works as a single-page app.
Text selection is now enabled on the "home page" content.
Author here :-) You access the exact same APIs with all 4 languages. In Python and Lua there are a few differences in how you should call an object method or how you can safely access user input properties (they are detailed in the Python and Lua tabs of the documentation). In JavaScript the API is 100% used the same way.
Text selection is now enabled on the "home page" content.