I don't see a chance to change this without custom coding. Limesurvey defines a response complete only when the survey is submitted. This is especially necessary for surveys that are in "all-in-one" mode. But also in Group by Group or Question by Question, there is no way to guarantee that all questions are answered (that the survey has really been finished) without the final "next" or better "submit" action.
However, you can go into the list of responses and mark a response as complete manually. This might not be what you are looking for, but I thought I'd mention it.
However, you can go into the list of responses and mark a response as complete manually. This might not be what you are looking for, but I thought I'd mention it.