Doesn't work in Scratch 3.0 yet, but I've got the go-to block implemented locally.
The button will light up when the gliding cat goes "over" it, i.e. when the distance between the two sprites is less than 30.
A bunch of other cats will become less ghosty when the mouse pointer gets closer to them.
There was also one check I couldn't find a use case for, but implemented anyways - the distance to a nonexistent sprite should be 10000.