- When I move the cursor all the way to the top, the screen starts to flicker.

Yes, annoying isn't it. It happens because the cursors are transparent images and when they move over the edge of other components they cause flicker.

- When I cut through a large pile of 'snow' (upwards) a line up black pixels went also upwards.

That's the 'bubbles' moving up because you erased the line, so snow has to fill that space so snow moves down and bubbles move up - unless I'm thinking of something else.
There is a bug with the snow somewhere. Sometimes piled up snow doesn't fall when it should do after erasing a line. it only happens near the top of the screen and I can't work out what's causing it for the life of me.

I've thought of adding AI to play against but it would have a major advantage in control but I suppose I could slow it down or something.

Anyway, glad you like the idea.

Peter