There's been a couple of my threads that got derailed, LOL, but they went into another thread that someone politely started (Tog for one on "Weird Foods"). I agree that sometimes it goes on TOO long when, a new thread should be started to keep the old topic going, but a little going "off" is fine & FUN.
John, I LOVED the idea of the birdbath

& you were not off topic!
Did you here the one about .......?

I remember few years ago at GW - 2 ppl were the only 2 posting on a thread for the longest time - it was irritating. Someone posted that they should just email or PM each other since it was just between the 2 of them. It NEVER got that bad here though.
There are a few "derailers" here

but usually it doesn't last long & it's usually funny. If it does go off topic, just post something & I'm sure they will stop.
I've seen Harvey stop people derailing a thread he started LOL. It worked!