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I saw a discussion once about how difficult the idea of an accurate progress bar really is. Note how so many things now take the easier approach of a circular or other shape that shows activity without being tied to any particular time prediction. My preference is a combo progress bar with actual throughput numbers/graph, so you can tell if there's really progress or some bottleneck that will make it longer.
A progress bar isn't generally hard.
What is hard is making it move smoothly while still only incrementing and getting the correct time. But I don't think there's a single person that still cares about smoothness except for the very extreme cases. By now, everybody has learned they don't work that way.
Anyway, about those extreme cases, just avoid filling the bar up to 99% on the first 10 seconds and leaving it there for the next 2 hours. Just do that and you'll be already better than Microsoft, so nobody will complain.
And Pojav Launcher for some reason makes the progress bar of the number of files downloaded, while ignoring the size of the actual files, so it is very uneven when hundreds of files download in one second, and 1 file takes 10 seconds to download.