Prior to starting work, there should have been a document signed whereby you listed all pre-existing IP. Mingling this code with the employer's app does indeed

Prior to starting work, there should have been a document signed whereby you listed all pre-existing IP. Mingling this code with the employer's app does indeed make it theirs, but not exclusively theirs. If possible avoid any cut-n-paste mingling, incorporate your IP as a separate re-write, and maintain a github history of your own IP for future reference to prove ownership.