The Commonwealth of Poland-Lithuania was, historically, inefficient and unequal. That brought its demise in 1795, after the third partition of Poland-Lithuania.
This map, however, shows a slightly different/alternate Commonwealth of Two Nations. In it, the Commonwealth started out entirely equally- Poland was not the superior member of the commonwealth, but instead all of its states ( as shown in the map ) had a saying in governing matters. There was no fixed capital ( the temporary ruling states kept shifting the capitals between each other; for example, for 4 years the capital would be Vilnius, then Warsaw for the next 4 years, then Kiev, and etc. ), and the ruler of the nation was usually elected from one of the commonwealth's states, and not some foreign nation. Due to its government's increased efficiency and general content among its people, the Commonwealth of Both Nations managed to survive up until modern day ( or at least 1910, as the map shows ), and preserve its borders relatively unchanged.
In this alternate timeline, the cultures of the Commonwealth's component states were generally preserved in their own territories, and population was the largest in Europe throughout most of the commonwealth's existance.