Thank you for the information. I was on the verge of suggesting exactly this feature before you posted about it, but I was too tired to write coherently. It’s not at all obvious to me that the little arrow thing is a button with a menu. (Especially since its tooltip is “null”.)
I think this is often the reason threads go off-topic. The poster might know they are going slightly off-topic, but within the realms of acceptability. It is not worth starting a new thread for the original remark. But then, unexpectedly, dozens of other people pile on, responding to that remark. It’s difficult to anticipate when a remark might blow up out of proportion.
A cool feature might be the ability to mark sections of your own posts as off-topic, so that if anyone tries to reply to your post without quoting an on-topic section of it, or quoting an off-topic section of it, they will receive an firm suggestion to start a new thread. In the editor, the feature could be called “off-topic reply barrier” or something like that, and applied the same way as emphasis.