In 2015 I spent over a week in Zagreb and a little time in Zadar--not necessarily representative of Split, I realize. I am sensitive to smoke and get annoyed if people smoke in non-smoking areas. I did not encounter that in Croatia, though it's probably the case that smoking is explicitly allowed at outdoor tables. I'm accustomed to heading straight indoors as a matter of course. It's not always a pleasant experience in warm weather, because a/c is not universally present. There can also be a bit of an issue with slow service if you're the only person sitting indoors while all the other customers puff away outside, but it goes without saying that a group of 15 people is not going to be forgotten! I'm a solo traveler and no ones top priority. And European waiters aren't good about checking to see whether you need more water anyway, even if you are sitting in a high-traffic area.
The place I've had the most trouble avoiding smoke at restaurants was southern France, down near the Spanish border. Last year two different restaurants refused to let me sit inside at lunchtime, though there were tables present. It was sit outdoors with the smokers or nothing, so I walked away. If I'd had time I'd have gone to the tourist office and complained.