Re-entering Schengen from Croatia on exactly the 90th day after Exit?

For the Schengen Area, your arrival and departure dates both count as full days. I guess it is the same for Croatia
If you left the Schengen Area on day 87 (with 3 days spare), that is also Croatia day one. So Croatia day 90 is Day 176 of your "90 days in 180".
Day 176 back to the Schengen Area ==> 88th day
Day 177 in Schengen Area ==> 89th day
Day 178 in Schengen Area ==> 90th day, you must leave again.
Day 179
Day 180
Day 181, Your first day in Schengen is now 181 days ago, and doesn't count. You now only have 89 days in the last 180, and so can in theory go back into the Schengen Area.

But, I really wouldn't recommend cutting it that close, you may have problems. And what if you are delayed leaving?
Safer not to re-enter until day 181, or 182. How about Bosnia?

Yeah, I think you missed that day 90 in Schengen is also Day 1 in Croatia, then Day 90 in Croatia also day 1 in Schengen, meaning that you do not meet the 90 days in last 180 days.

You're not worried about the border guards rolling their eyes and muttering things about you, you're worried about being denied entry. If you follow the rules then they won’t have a reason to deny entry.

Only you know why you're moving in and out of the Schengen once your 90 days is up. What you're doing is trying to game the system, so don’t be surprised if the border guards question you very closely.

If being in the Schengen is so important to you apply for the appropriate visa - or bite the bullet and return home for a while.

I'd rather avoid flying to UK or some other non-schengen place

I think that is the best solution. Lots of flights from Croatia to London, spend a few days there, then get a train to Paris.

buses to france are cheap from Croatia.

I would not want to take a bus all the way from Croatia to France. That will take forever!
I looked on https://www.flixbus.com/ Zagreb to Paris, fastest 23 hours, cheapest €56.
Easyjet, split to London, cheapest €56.86

For the last three summers I've left the Schengen Zone on Day 89. I got some extra attention in the airline check-in line at the Barcelona airport (outbound) last year, which I guess might have been due to the timing, but I never found out for sure. No one ever said anything to me about my time in the Schengen Zone.

I do think scheduling yourself tightly is risky. I became ill on Day 84 in 2015 and only recovered enough to travel onward on Day 89.

I've been researching the same thing and came up with a potential solution. There is a strip of Bosnia north of Dubrovnik (Neum) that cuts Croatia in half. If you can stay there for 3-4 nights, that buys you a few days out of Schengen that won't count in Croatia. Sounds like the borders are not highly checked, so you may have to make sure your passport gets stamped going in and out.

Maybe some one can validate if this worked for them.

Can you book a ticket THROUGH Schengen and then just not leave again? Are you allowed to pass through?

Maybe you should just get the appropriate paperwork.

I traveled through that strip of Bisnia a couple of years ago and the bus was boarded by immigration officers checking id, twice.

Karen, anywhere in Bosnia-Hercegovina should work, but I'd head to Mostar or Sarajevo unless there's something interesting in that little costal strip. Assuming I was in southern Croatia, that is.

Thanks everyone!

I guess i was confused about how it worked. I thought that as soon as i left the 90 days started counting down, not 180 after day 1 entry.

I think i will try to visit Mostar for a week to get the extra 7 days needed. I prefer not to fly because it is too quick. I enjoy the long bus rides because i can see the landscape and people changing. It feels more like a journey.

I would get a long-stay freelance VISA, however they require you to make 2,500 euros a month. I don't make nearly that much online, but still enough to buy food and live comfortably in cheap airBnbs and hostels.

Can you book a ticket THROUGH Schengen and then just not leave again? Are you allowed to pass through?

Astorienne, nationalities who are subject to the "90 days in 180" rule have their passports stamped on entry to and exit from the Schengen Area, which is how they keep track and catch overstayers.
If you are changing planes at an airport, you stay "airside", do not officially enter the country, your passport is not stamped and so it doesn't count.
If you are travelling by land or sea, or if you exit the airport, your passport will be stamped for entry, and they will (probably) spot on exit that you overstayed. You risk a fine and a ban from re-entering the Schengen Area.