In its 25 year history, Croatian First League title was won only by three teams: Dinamo (18 times), Hajduk (6 times) and Zagreb (once). Considering Zagreb is on the verge of being relegated to third tier, the league itself can be compared to Scottish (Celtic, Rangers) or Ukrainian (Dynamo, Shakhtar) leagues.
Interestingly enough, Rijeka should have won the 1999 championship but was robbed of it by a corrupt system which involve the country's president himself. A short quote from Wikipedia says it all (note: Croatia Zagreb afterwards changed its name to Dinamo Zagreb):
In the final round of the 1998–99 season, a refereeing error denied Rijeka their first championship title. With one match to go, Rijeka were one point ahead of Croatia Zagreb, needing a home win against Osijek to secure the title. With the match tied at 1–1, in the 89th minute, Rijeka forward Admir Hasančić converted a cross by Barnabás Sztipánovics. The packed Kantrida erupted in celebration. However, to their dismay, moments later, assistant referee Krečak raised his flag and referee Šupraha disallowed Rijeka's winning goal for alleged offside. Following an investigation, 3D analysis revealed that Hasančić was not in an offside position (clear by 12 centimeters) and that Rijeka were wrongfully denied their first championship title. An investigation by Nacional revealed that Franjo Tuđman, the President of the Republic of Croatia and an ardent Croatia Zagreb supporter, earlier in 1999 ordered the country's intelligence agencies to spy on football referees, officials and journalists, with the aim of ensuring that the Zagreb club wins the league title.
https://en.wikipedia.org/wiki/HNK_Rijeka
Rijeka went on and fought with Osijek for that 3rd place in the following years, had some success in winning the Cup two times in a row (2005, 2006) but was face to face with bankruptcy around 2011 and 2012, just like some other domestic clubs; most notable was Varteks - they were relegated to the lowest Croatian football level due to financial problems in 2012.
The arrival of Italian businessman Gabrielle Volpi and his Social Sport foundation came at the last minute. Bringing in his longtime oil business partner and an avid Rijeka fan Damir Mišković in the club, began the process of restoring Rijeka to their former glory, with Slovenian manager Matjaž Kek. The financial backing is much more alike Monaco's instead of Man City or PSG in terms of bigger budgets but still realistic transfer targets, focusing on player personality first, buying mostly Croatians or players from neighboring countries. Most notable player sold was Andrej Kramarić - bought for €400k from Dinamo and sold to Leicester City for €11.5m.
In just five years, a modern football camp worth €30m that included a 6000 capacity Rujevica Stadium was built - to serve both as a backup stadium while the club's old Kantrida Stadium is rebuilt, the club won Croatian Cup and Supercup trophies, qualified for Europa League group stage twice - with some notable results: * eliminating Stuttgart in 2013-14 EL playoffs on away goals by scoring the equalizing goal in 93rd minute in Mercedes-Benz Arena * 3-1 and 2-0 wins against Feyenoord and Standard Liege in 2014-15 campaign, along with a 2-2 draw vs champions Sevilla with the Spaniards equalizing in 91st minute (all home games)
After four years of finishing 2nd and being shadowed by a much stronger Dinamo side - player-wise, financially and logistically in terms of referee bias and media support - Rijeka secured this year's title in front of their own fans, with one game to go. The celebrations will continue through the night in the city and thousands of supporters won't be getting much sleep tonight! The final round, when the trophy will be given, is scheduled for next weekend and will be played vs Dinamo on their pitch. Talk about irony. The funny thing is - Dinamo has no major support anymore. Their ultras group The Bad Blue Boys boycotted the matches for quite a while until only recently, and they spend some time during each match chanting against Dinamo's controversal boss Zdravko Mamić, who faces trial for tax evasion in many of the club's major transfers including Modric, Kovacic and Brozovic sales.
On the other hand, the atmosphere around Rijeka is at its very top. The fans follow the team in huge numbers in away games and they are one with the players and the staff.