>  I wanted to hide the
>  fringe marks conditionally while they align with the real window-start
>  and window-end (they're not useful then),

Huh. Of course, what's an apparently viable workaround?  Another idle
timer! Yay!

Version 2 attached - made it a global minor mode.

Probably lots of issues, most major apart from previous mail's
"am I using fringes and overlays right" question:

really should use separate overlay for each window (maybe weak hash
table needed, or is there a way to hang things off windows I've
missed?), right now, only shows in current window, which is a pity
because it'd probably be particularly nice for scroll-other-window
situations.

nongui terminals and gui terminals without fringes, could use
alternate highlighting strategies I guess.

If you scroll fast, then what's fringe marked is of course not the
immediately preceding window position, but the one from when emacs was
last idle.  This is both desirable and undesirable, depending on
whether you consider a series of consecutive scroll commands as one
overall scroll operation or separate scroll operations, so not sure if
should be fixed, maybe a post-command-hook could be used so that
depending on user prefs, page-scrolling could fringe mark anew each
time while scroll-bar-scrolling only after the lag, or whatever makes
sense.