Emits after the previewed panel changes, so mounted slots can redraw.
The previewed panel, or null when there is none.
The saved row this panel is an edit of. That row is left off the preview form.
Shows spec on the preview form, or nothing when it is null. With a component the
panel draws that component; without one it draws a placeholder.
A panel the placement dialog shows on a real form before anything is saved.
The dialog provides one of these to the form it renders as its preview. Every place that collects a form's contributions adds this registration, so the form itself decides where the panel draws, what it hides and what rail item it gets. A form outside the preview has no provider, so nothing else ever sees it.