Preemptive multitasking, with no memory protection. I think they leveraged this to do stuff like update the appearance of buttons on click in the higher priori

Preemptive multitasking, with no memory protection. I think they leveraged this to do stuff like update the appearance of buttons on click in the higher priority event process (and then send an event), to give a more responsive feel to the UI. I also remember Mosaic once had a use after free bug on graphics contexts and would occasionally render a web page in another window.