Continuing my drive to clear up some of the ancient backlog of word suggestions...
I could find
flyout in only one dictionary:
Wiktionary. Wiktionary gives two meanings:
1. (baseball) Alternative form of fly out
2. (computing, graphical user interface) A popup menu that branches off some other visual element.
In the News on the Web corpus the baseball usage seems to dominate. E.g. the
Cincinnati Enquirer last month: "Kansas City avoided more damage when Starling threw out Lindor trying to score on a flyout."
In the computer context I doubt the word is known by many users of computers but, as TRex said, the word is used quite frequently by software developers. However we've all seen flyout menus. In the screen grab below, from the Windows File Explorer, the list of options at the right, beginning with Firefox, is a flyout menu.
Despite its only being in Wiktionary, the word is used often enough to warrant acceptance as a rare word.