Plot value not restored correctly when interacting with multiple pointers

If the mouse is within the radius of multiple pointers and moves outside of one pointer's max radius, then the plot value will be set to null instead of according to the remaining active pointer.

Example (notebook):

viewof value = Plot.plot({
  marks: [
    Plot.frame(),
    Plot.rect(["foo"], Plot.pointerY({y: d => d, maxRadius: Infinity, fill: "orange"})),
    Plot.dot([{y: "foo"}], {y: "y", tip: true})
  ],
})

Steps:

1. Move the mouse into the frame on either the left or right side so that value reads "foo".
2. Move the mouse towards the dot until the tip appears; observe that value reads {y: "foo"}.
3. Within the frame, move the mouse away from the dot until the tip disappears; observe that value reads null instead of "foo".

Demo:

Start with the Observable notebook linked in the issue and trace the interaction entry points used by Plot.pointerY and the tip from Plot.dot. Reproduce the pointer movement sequence, then verify that leaving the dot keeps value set to "foo" while the pointerY mark remains active.