Torrent stalls while app window is open

I'm using Bitford on a Chromebook. I've noticed that the transfer rate dwindles to zero on some torrents. When I close the app window, Bitford keeps running as a background page (in the Chrome Task Manager) using about 500MB RAM and 100% CPU. When I reopen the app window, the torrent has made progress. The transfer rate starts off high, but again dwindles to zero over about 60 sec.
I'm attaching a screenshot showing a pretty long list of peers, but all at zero. The Pieces graph looks a bit odd to me, as they usually go straight from grey to green, only very briefly other colours. The graph you see was stuck like this: no movement.
Sorry if this is vague and non-technical: I'm not a coder. If you need more details please let me know.

Start by reproducing the stall on a Chromebook while the app window is open, then compare CPU, memory, peer activity, and the Pieces graph after closing and reopening the window. Done means the torrent continues transferring instead of reaching zero after about 60 seconds; the issue names no files or tests, so locating the responsible code will require tracing the app-window and background-page behavior.

Written by the indexing model from the issue text.