Question

Is there a rule of grammar that prevents the separation of the present progressive? I was told that the "to be" needs to be immediately followed by the -ing verb. Ex. "She is dancing." but that it is incorrect to separate them with a modifier such as "She is still dancing". and that it is correctly written "She still is dancing." It sounds wrong that way to me. I just want to know what the proper rules of English writing say about this. It is proper to separate them with a negative. She is not singing. I'm just curious. Thanks