Charles Mingus was onstage with his band when he was told that Lester Young had died. He called a short minute break and while his band went and got themselves a drink, wrote and arranged (for an 8-piece band) "Goodbye Pork Pie Hat"[1] in tribute. They came back on stage and played it immediately.

Keith Jarrett wrote all the music for a tour of Japan on the plane there. That became the album "Personal Mountains"[2]. Apparently he literally had all the meal trays around him open with scores on them and was scribbling away the whole way.

[1] One of the all-time classic jazz ballads. https://www.youtube.com/watch?v=CWWO_VcdnHY

[2] https://www.youtube.com/watch?v=L8rokRx3lnY