Of course, there _is_ a line, and Russia will probe to see where that is, but they won't cross it.
It's yet another reason why NATO is so firmly bound to supporting Ukraine. Before the opening of direct hostilities there, Russia engaged in all sorts of shit in the west with no consequences. Cyber attacks and surveillance stuff, but actually doing things like brazenly poisoning people on NATO soil and in the process killing UK civilians. Or shooting down civilian airliners. And never paying any consequence because we were afraid of war.
Well, now there's a war, but we don't have to be directly involved with the threat of nukes and cruise missiles hitting our cities...
The one place where I do see risk of the line being crossed is in the Black Sea. I'm disappointed that Turkey and NATO have not taken a firmer line. I hope/expect to see a stronger response to events like what happened in the port on the Danube a couple days ago.
It makes no sense for Russia to be allowed to control the waters outside of its territory on a sea where the coastline is filled with NATO countries.
If there is a serious flare up, it will be there, and naval.