When the file loads, it's collapsed automatically
if the user has reviewed it already.

Unfortunately, this only happens when the component
mounts.

To make the component reactive - specifically in
file-by-file mode - we add the check for the collapse
state into a watcher on the file ID.

Critically, we leave the `mounted` check in place, too.
The first time the component is mounted, it has the
initial file already present - the watcher won't trigger
on that first render.