Comments should be English sentences, so including capitalisation and punctuation. Same for the other comments.

static void

See https://wiki.blender.org/wiki/Style_Guide/C_Cpp#Naming on naming of function arguments.

Having said that, I think it's better to have this function return a ListBase /* NlaStrip */ instead.

Many comments refer to tracks, when they should refer to strips.

This means that the if and else blocks should just be their own function.

if the strips are stored in order, you should be able to break here.

This is not what happens in the code. The comment suggests comparison between distances, which doesn't happen.

use LISTBASE_FOREACH

This considers each and every strip that's after the current frame, and if it's preceeded by an extending strip, returns that preceeding strip. This will produce false positives, namely when the actual strip just left of the viewport is NOT extending, but some later strip actually is.

I think the code will clean up a bit if you defer the ListBase construction to a spot later in the function. That way you can do something like this:

if (BLI_listbase_is_empty(&nlt->strips)) {
  ListBase empty = {NULL, NULL};
  return empty;
}

NlaStrip *first = NULL;
NlaStrip *last = NULL;

// ... Do stuff, set first/last

ListBase visible_strips = {first, last};
return visible_strips;

This also makes breaking up this function into smaller bits easier as you can just do something like this, without having to bother passing first and last around:

if (has_strips_within_bounds) {
  return the_first_function(...);
}
return the_other_function(...);

What I mean is that the need to comments blocks of code, to explain what they do, can be used as an indicator that that particular block should actually be in its own function. Looking at the maximum indentation level of the code below, I think that's actually a good idea.