Prevent user to delete decoration

I want to replace the first character of the editor with a widget, for that I am using replace Decoration , now I want that user should not be able to delete that first character and user should not be able to write before that first character and it should not be copied as well

A change filter can be used to block certain kinds of changes. I’m not sure about preventing copying. Possibly a transaction filter that prevents the first character from ever being selected is what you want.

Thanks , transaction filter helped