This condition is too complex, and on top of that the result is used in another negation. It's better to transform it into a separate function. That can then check various sub-conditions, maybe have some comments to explain what's going on, and use early returns to short-cut the evaluation. You can then call the function where it's needed, instead of declaring the boolean at the top of the function but using it in the middle.

As a rule of thumb, try to avoid double negations. The existing code is also double-negating, using if (non_solo == 0), in other words "if not non-solo". If that were on my plate to review now I'd also not accept it ;-)

The condition also produces a compiler warning:

blender/source/blender/editors/space_nla/nla_draw.c: In function ‘nla_draw_strip’:
blender/source/blender/editors/space_nla/nla_draw.c:424:55: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
  424 |                               (adt->act_track == nlt) &&(adt->actstrip != strip));
      |                               ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~