Cancel must stop network bytes, not just the label
Context
ReviewTube upload UI offered a cancel control during multipart transfer.
What went wrong
Cancel set an aborted flag while multipart PUTs kept flying. The button looked live and did not change the world.
What we changed
I track active XHRs and abort them immediately. If the master file already sealed, cancel does not destroy a good complete.
Result
Cancel kills bytes in flight. Retry cannot create a silent duplicate from a half-killed transfer.