next-error-highlight = t - highlighting doesn't stay until you move to the next locus

[Drew Adams]

The doc string says:

"If t, highlight the locus indefinitely until some
 other locus replaces it."

I wish it were so. However, the code says this:

(if (or (eq next-error-highlight t)
        (numberp next-error-highlight))
    ;; We want highlighting:
    ;; delete overlay on next input.  <---------- NOT GOOD
    (add-hook 'pre-command-hook 'compilation-goto-locus-delete-o)
...

What I want: Leave the highlighting in place, until I ask to move to another
locus (and then highlight that one) - just as the doc says.

What happens: The highlighting disappears completely upon the next event.
For example, if I use pop-up-frames, and I click in the source buffer
(frame), the highlighting disappears. If the source-buffer frame is slightly
behind the *grep* frame, then I can't see where the hit is - as soon as I
click the frame, the highlighting disappears.

IOW, why add the deletion to pre-command-hook? It should, as the doc says,
be associated with the request to move to another hit.