[WorldMaker]

I had great results using a circular progress bar instead of a linear one. If you allow the tail to "catch up" towards the head, but the head always moving forward (rotating clockwise) you can allow the progress bar to shrink without losing the illusion of forward progress. In the worst case where lots new tasks/sub-tasks are discovered on the fly, it acts directly as its own "spinner".

It's probably easier to visualize than describe, but I have longer descriptions on the thinking behind it on my blog/GitHub repo. Unfortunately, my demo site succumbed to JS CDN bit rot and I keep forgetting to update the demo to something more recent.

(ETA: Forgot the link: https://github.com/WorldMaker/compradprog Also thought I could point out that the idea mostly jives with the findings in the paper here.)