Stop the reveal widget caching hidden content

Previously, when displayed content is hidden by the reveal widget there
was an optimisation such that the content was retained in the DOM but
hidden using CSS, so that it could be shown again quickly.

It turns out that a sideeffect of that optimisation is that clicking
through all the sidebar tabs leaves them all active, so that they all
need to be refreshed whenever a character is typed in an editor.

This commit suppresses the optimisation, so that hidden content is
removed from the DOM and the render tree.