Often a flat screwdriver or old credit card can pop the spline into place.
What it gives you an excuse to buy: The materials for this repair are inexpensive and will last for more than one window, which means you’ll have them on hand the next time a screen is damaged. Often a flat screwdriver or old credit card can pop the spline into place. But a spline roller makes things go smoother and decreases the risk of damaging the spline during installation. And a spline roller lasts forever!