In 1573, following the death of the Polish ruler Sigisimund III Augustus, a French envoy was sent to Poland to negotiate the election of Henry of Valois, future Henry III of France, on the Polish throne, in exchange for military support against Russia, diplomatic assistance in dealing with the Ottoman Empire and financial help. On 16 May 1573 Polish nobles elected Henry, as the first elected monarch of the Polish-Lithuania Commonwealth.
Most of Henry's reign was spent traveling to the country following the election. On May 30, 1574, King Charles IX of France died without an issue leaving the throne to his brother, Henry. Almost immediately he departed, sparking a constitutional crisis within Poland-Lithuania.
What if Charles IX's mental and physical health improves, preventing his untimely death? What does Poland-Lithaunia look like with Henry on the throne?