A periwinkle flower!
Well this is surprising, I'd never even heard of the Periwinkle (as a plant, rather than a sea snail) and now I have a flower that looks very much like a periwinkle flower. But it is not on the plant in the recent article Maybe Periwinkle? .
Anyway first here is the flower. It is quite small no more than a centimeter across. It is the only one on the plant and there are no obvious other buds either.
So this is another spreading plant with much smaller leaves that are variegated light green and white. It is by the side of the house at the bottom of the picture below.
It is also spreading by rooting wherever it touches the soil. This one looks easier to keep under control, but only because it is more accessible and not mixed up in any other plants. Since the photo was taken I've removed quite a bit of the plant away from the flower.
Since I didn't buy it I am unlikely to ever know the exact variety, but searching on the web I do find pictures that are a bit similar although nothing that exactly the same.