I had to add tread height, to make an old stair come out even when adding an insulated floor below (see How can I deal with a short bottom step on a stairwell? ). Now I've got a gap on the top tread's sill nosing. What type of product might work? "Bondo" is often used for such things, but it may be too stiff and crack over time:
What might be both structural, and form a neat enough edge? The stairs will be painted, then have carpet runners. Note that each stair from here down was raised by 1/4" more than the prior one, averaging out the short bottom step.