libace-flreactor-6.4.5 6.4.5+dfsg-1build2 (arm64 binary) in ubuntu focal

 Recognizing the need to write reactor-based GUI applications, the ACE
 community has created several reactor extensions for use with X
 Window System. Each of these extends the ACE_Select_Reactor to work
 with a specific toolkit. By using these reactors, your GUI
 application can remain single threaded yet still respond to both GUI
 events, such as button presses, and your own application events.
 .
 The ACE_FlReactor integrates with the FastLight toolkit's Fl::wait()
 method.