Croatia: Adriatic Coast, Southwest of Hungary, Capital Zagreb

The country matching all three criteria is Croatia (Hrvatska).

Verification

• Adriatic Sea border: 1,777 km coastline (mainland + 1,200+ islands)
• Southwest of Hungary: Shares 348 km border with Hungary (north/northeast of Croatia)
• Capital: Zagreb (population ~770,000; metro ~1.1M)

Additional Context

• Independence: 1991 (from Yugoslavia)
• EU member: Since 2013; Eurozone/Schengen 2023
• Geography: Pannonian Plain (east), Dinaric Alps (west), Dalmatian coast
• Population: ~3.8M
• Language: Croatian (Latin script)

Neighbors

Slovenia (NW), Hungary (NE), Serbia (E), Bosnia & Herzegovina (SE), Montenegro (S), maritime border with Italy.