I have written this years ago ... and have used it for years. I looked around and did not see anything similar
Command Name:
narrow-to-region, & narrowTog
Most recently tested on:
SlickEdit 15.0
Originally written for v6.0
Description:
Extends VSE's selective display feature to mimic Emacs' narrow-to-region functionality.
Select a region of your buffer, then invoke narrowTog. narrowTog will hide the areas above and below
your selected region. Invoke narrowTog again to unhide the area (restoring the line selection).
Installation:
Save the macro file locally and load it via Macro > Load.
Also attaching a pretty outdated bmp image that I use in a toolbar button bound to narrowTog