What I did
This restore display of nested pull progress events. Also

• refactored event management code for readability with newTask for first event and update to manage state change
• added management of multiple parent resources (service images pulling a shared layer). Only the first declared parent can update task state, to avoid status to flicker and enforce the "Done" event is not overridden

demo: https://asciinema.org/a/KVW9EVGBLTEZbsOeL6UgZDmyg
notice f626.. layer being downloaded by both images and successfully reported as pulled on completion

Related issue
fixes #13443

(not mandatory) A picture of a cute animal, if possible in relation to what you did