[Karl Wescott]

There is hijacking... and there is subject drift. The first priority should be (and generally is) to answer the question or resolve whatever initiated the post. Hijacking by changing to an entirely different subject should be avoided as it really needs its own thread.
Subject drift can be fun. A question such as "Why does my 1936 pickup have a garnish molding on one window and not on the other?" Once answered "Early 1935 had garnish... late 1935-37 did not". This could easily drift to when the change occurred, why it occurred, and obscure details (perhaps the color of the primer on the inside of the molding).
Suggesting to a poster that a comment is a hijack is imho not really polite and is unnecessarily confrontational. A more positive comment may be that the (hijack) subject warrants its own thread, or to create a new thread if the subject drift has enough relevance to both be preserved and expanded upon, or identify drift that is not significant with a note on the title of the response "Sidebar ..." As I have done.