When a read-only field (which has its own canvas) is updated by the
sandbox while its page isn't rendered, showElementAndHideCanvas
isn't called, so once the page is finally rendered the field still
shows its outdated canvas instead of the new value.
Replace the imperative canvas/element toggling with a `sandboxModified`
class, set from the annotation storage both at render time and on
sandbox updates, and let the CSS show the element and hide the canvas.
ecd43bc8e1