1. a terminology issue: I was thinking of the Polish-Lithuanian Union (1385-1795), of which the Polish-Lithuanian Commonwealth was only the last inception (1569-1795);

2. the Jagiellons themselves, despite dynastic succession, had to face election:

In 1505 Sejm concluded that no new law could be established without the agreement of the nobility (the Nihil Novi act). King Alexander Jagiellon was forced to agree to this settlement. The Sejm operated on the principle of unanimous consent, regarding each noble as irreducibly sovereign. In a further safeguard of minority rights, Polish usage sanctioned the right of a group of nobility to form a confederation, which in effect constituted an uprising aimed at redress of grievances. The nobility also possessed the crucial right to elect the monarch, although the Jagiellons were in practice a hereditary ruling house in all but the formal sense. In fact, Jagiellons had to give privileges to the nobles to encourage them to elect their sons to be the successors. Those privileges reduced king's power. King Sigismund II Augustus was the last of Jagiellon dynasty; he had no sons. The prestige of the Jagiellons and the certainty of their succession supplied an element of cohesion that tempered the disruptive forces built into the state system.