Document possible stumbling block for later
Clarify sandwich hit testing a little
Don't bubble synthetic event ignored by canvas.

We should stop bubbling of any synthetic event we dispatch to a canvas layer if the canvas doesn't respond to it, lest listeners on the window see > 1 synthetic event for every real event.
Hit should hide event from viewport drag handler

[#57815384]

By listening for viewport dragging on the <div> containing the svg/canvas sandwich layers, mouse events on atoms were bubbling to our viewport handler. This meant that dragging an atom with viewPortDragging = true dragged the atom across the model *and* dragged the model (so the atom went zooming at 2x speed).

By instead listening for viewport dragging events on the bottom-most <svg> of the sandwich, we get essentially the behavior we had in the svg-only view.