Cool new Free software

There is no "the kernel" versus "the user process" when it comes to doing things -- there is just a CPU running some code, with different memory access rules. Presumably the way you would tell the kernel about this cool shadow and how to draw it is that you would provide some shadow drawing machine code, and then when the mouse pointer moved the kernel would set up some registers etc. (a "context") and then run this machine code. But now we have just described exactly what user processes are...

When a user process blocks, it doesn't actually sit there as an entity taking up space. It gets converted into a callback attached to whatever event caused it to block.