I came to check out the lupines since I was in the neighborhood. I looked around to see if there were any established looking ones and saw this one blooming.
It's a pretty small plant, maybe 8 or 9 inches tall (the pic with the ruler is up to 5.5 inches). But this one looks robust and seems to be getting a decent amount of sun.
Last picture is of another nearby plant, also looks established.
Made an impromptu visit and ran into this flower.
Seems like Geranium maculatum forma albiflorum.