This bug report was confused from the beginning. As Tim said, how a progress bar's value is calculated is up to the app. Some progress bars are used for downloads, and some aren't, and the toolkit does not know -- and has no reason to know -- which is which.
If particular download progress bars are inaccurate, they should be reported individually, for example bug 1227777, bug 1429022, and bug 1450144. Percentages in progress bars is a separate issue that I've added to the toolkit specification.
This bug report was confused from the beginning. As Tim said, how a progress bar's value is calculated is up to the app. Some progress bars are used for downloads, and some aren't, and the toolkit does not know -- and has no reason to know -- which is which.
If particular download progress bars are inaccurate, they should be reported individually, for example bug 1227777, bug 1429022, and bug 1450144. Percentages in progress bars is a separate issue that I've added to the toolkit specification.