It seems that the most of the old
K. und K.* lands are free now - free movement without any control between CZ, SK, AT, HU (and AFAIK SI). The news mentioned "citizens of" but if it's really no-control, there is no enforcement (once you are in) RS has the obvious immigration controls, it being outside the EU, but otherwise good to go as is Montenegro.
The "bubbles" seem to grow together ^
* that's Kaiserlich und Königlich (Imperial and Royal) ie. the Austro-Hungarian empire for the historically challenged 