Assuming Croatia gets independent under Jelacic post 1849... so between 1849 and 1866... how do you think Hungary will keep the relations with Croatia? Hostile or more developing?
Considering Croatia has no claim in Hungary but Serbia does.
Depends greatly on Serbian-Croatian relations. Lets assume that Croatia gets independent with no border changes toward Hungary. That puts a significant serbian population under Croatian rule as the territory between the Sava and Danube - across Belgrad - would be part of Croatia. If they have sour relations and Hungary renounces its claims on Croatia - likely in this scenario - I think Hungary and Croatia will be natural allies to each other. To demonstrate: OTL after WWI Croatian ustashe's had training camps in Hungary. Croatian nationalism could easily have this result. Or they could try to make common cause with the serbs - likely they try at first but as stated above, nationalism can easily sour this so I have little hope it working in the long run. The problem is I dont know enough of Jelacic to guess which direction he would go. It also wont help when serbian serbs will start to demand annexation of said territory of Croatia.
As I see it right after independence the main focus of Hungary will be to regain its lost territories - Croatia not included of course. Hungary also needs good relations with Croatia as the animosity with Austria makes it the most likely route for hungarian grain through croatian ports. Croatia too will have interest to have this hungarian trade go through them.
To sum it up: My guess is it doesnt start great however as hungary is focused elsewhere and needs Croatia for its trade and I expect Croatian-serbian relations to sour pretty quickly they will soon become allies or something close to that.