Narrow your view on multiple-cursor marks

The discussion in the comments of this post is great. It reveals a couple of ways to narrow your view, in a few frameworks. In particular it reveals that in multiple-cursors, all it takes is a call to mc-hide-unmatched-lines-mode.

(mc-hide-unmatched-lines-mode &optional ARG)

Minor mode when enabled hides all lines where no cursors (and
also hum/lines-to-expand below and above) To make use of this
mode press “C-‘” while multiple-cursor-mode is active. You can
still edit lines while you are in mc-hide-unmatched-lines
mode. To leave this mode press or “C-g”

Just be sure to exit this mode before closing Emacs as it is a little confusing to return to nothing.