Much better stated than what I tried to say.
The only thing I would do differently is, if the triggering action is taken by a successfully hidden character (who remains hidden through til the completion of the action), it could constitute a full action. The main reason for this is to avoid giving an extra free attack at the beginning of combat. (eg Rogue snipes enemies. Combat begins. Initiative is rolled. Enemies are surprised. Rogue and allies all act again, but enemies cannot act on their turns.) No double-dipping on surprise.
Keeping in mind that my target audience was my players, so it wasn't intended to be something that had to be perfect, here's my intent:
Absolutely, positively, never-ever can an attack occur outside of combat. If the monsters got that on the PCs, they'd scream bloody murder, and rightly so. In your example, the triggering action would be the rogue starting the attack -- it's not really an "action", but it's close enough for the intent. As soon as he decides to attack, initiative is rolled. If the enemies can't see him to know that an attack is imminent, then the PCs have surprise and the rogue's attack gets all the benefits of such. If none of the other PCs can see the rogue or have any way to know that he intended to attack at that point, I might rule that they were surprised, as well (I've had some groups where it wouldn't be out of the question for the rogue to do something to screw over the rest of the PCs). Really, though, I'd probably give the group the benefit of the doubt on having a signal set for starting combat so that everyone could just roll initiative and it made narrative sense (to an extent) that the fighter might actually beat the rogue to the punch (i.e. roll higher).
Now, something like quaffing a potion, casting a buff spell, etc. could certainly occur outside of combat, depending on circumstances. I might even let the PCs trigger a trap on their enemies outside of initiative, if it could conceivably be perceived as not suspicious enough to trigger combat stances -- the enemy would probably be on general guard enough to negate any chance for surprise during that "scene", though.
The "triggering event" for writing up that little blurb was when the PCs, who are generally stealthy except for the cleric, were trying to surprise a guard behind a door. The rogue had managed to sneak a peek into the room without breaking stealth, and closed the door again. Everyone gathered outside the door with great expectations. The cleric rolled a "1" on his stealth, but wasn't actually planning on doing anything on the first round, due to space constraints. The "1" was low enough that, even through a door, I felt compelled to say the guard had cause to be aware someone was out there. It could be argued that a solid door would have stopped the sound, but 2:00 AM in a monastery dormitory (PoA) is relatively quiet when a bodyguard is standing watch over their master, so I stand by that for purposes of organizing five PCs including multiple explanations to a cleric who just wasn't understanding "the plan". There were three separate, but important things that caused confusion:
First, many of the players kept wanting to apply the 3E concept of "flat footed", despite numerous clarifications that it doesn't exist in 5E. After the ensuing discussion, this is now crystal clear in everyone's mind.
Second, I didn't call for any stealth rolls until the PCs were in position and wanting to attack. This was my mistake. I should have called for the stealth rolls when they were moving into position. The "1" would have been obvious to everyone else. Even if I'd rolled for the PCs behind the screen, they would have at least been cognizant of the chance of alerting the guard. That stealth failure would have happened outside combat, and negated the opportunity for surprise. In this case, the guard would have held his post, rather than move to investigate, so no need to roll initiative (no engagement), but it
could have been the "triggering event".
Third, the players thought they needed to hold initiative to wait on the rogue opening the door (thinking that the rogue opening it stealthily would be what gave them surprise). They were very, very concerned by the potential logical problem of an unsurprised guard winning initiative and having his own attack prepared for when they opened the door -- that just broke any suspension of disbelief. In my defense, that's not how I planned to handle initiative, but I didn't have anything resembling the formal notion of a "triggering event" to explain to them; it was conceptually there, but not full enough to avoid the chaos of the first two issues.
Anyway, the end result was a bit of thoughtful DM time and that blurb. There are an almost infinite number of permutations around how combat can start. That's probably why WotC punted on the whole thing in 5E. They should have given more of a framework, IMO. So far, I'm happy with my ruling. It's been in place for a little over a month (verbalized before being written), so we'll see when a curve ball happens.