In bug 1204254 we are making service worker interception pass the body stream incrementally instead of accumulating the entire thing in memory. Unfortunately this means we cannot reliably determine the total size of the stream which makes progress events slightly less useful. (You can still see progress, but not total progress.) We could make this situation better if we implemented bug 1388773, updated our various streams to support the new interface, and then used it in interception code.
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