I tried to check this with vaping recently - inhaled, put on a standard surgical mask, exhaled. I think that’s a reasonable model of the airflow using a surgical mask. The vast majority (probably >90%) of the vapor came out of the left and right sides of the mask and next to my nose.
A very small amount diffused out of the front, but not much. There’s simply too much resistance, even with a thin 3-ply layer of material, for the air to go through the mask rather than around it, unless the mask is actually directly sealed to your lips/face.
That’s for regular air flow. I would imagine for sneezing (a mixture of air flow and droplets moving relatively fast) that a significant proportion of the air droplets get slammed into the mask and absorbed there owing to their momentum, with a lot still going around the sides and being redirected. Nonetheless, I’d expect that any particles/droplets would be absorbed by the mask/face to some degree while passing through the small channels between the two…so probably better than nothing. And the redirection of the airflow by the mask toward the user probably reduces the “spread” of a sneeze/cough to some degree.
I should note that I have a beard, which probably affects the results to some extent.