I was making a selection marker decal and to make it appear smoothly, I noticed a broader issue when rendering alpha-0 and alpha non-0 pixels in the shader.
I had to adjust my frag shader to draw opaque pixels in a separate pipeline from semi-transparent pixels:
Later, I encountered a smaller issue of y-sorting it correctly with respect to the environment (buildings, pavement) and the pawn.
After fixing both issues, the final version looked smoothed enough:
