What the schematic is to draw picked out, for a reason the draw block cannot know itself: at
present the waveform whose name the pointer is resting on, which only the UI layer knows about
because only it knows a simulation is running. Reaches the draw block the same way `overlay`
does - as an argument to SheetDisplay.view - and never as a message.
**Derived on every render from the state that says why, and never stored.** The highlight used
to be painted by the mouse handlers, which wrote the colours into the symbols and wires and put
them in the sheet's selection, and then unwrote them on the way out. Every one of those corner
cases is a transition that can be missed or arrive in the wrong order: a hover whose mouse-out
never came left the colour on the canvas, a repaint after it left the wire the wrong colour,
and ids of a simulation older than the sheet ended up in the selection where Delete could act
on them. As a function of the model there is nothing to put back.
Issie