Cancel button for shared history import doesn't seem to work

I mistakenly tried to import a very large history and would now like to cancel the import - there's a 'Cancel' button but it's greyed out.


Start by reproducing a shared history import with a large history and observe the greyed-out Cancel button. Trace the import UI and cancellation behavior; done means the button is usable when an import is in progress and cancels the import.

Written by the indexing model from the issue text.