Thanks @ashley.benson.tait and @J805 for your replies.
I’ve tried another app and the normal visual elements show, so it looks like the problem is isolated to this particular app. I’ve uploaded a screenshot showing again that there are no visual elements and for some reason there’s now a box with my email address in it - no idea what’s going on.
I can still use the editor as usual otherwise (create and edit workflows and elements, and copy and paste them, but can’t create new ones).
I tried to copy my app and it showed the same behaviour - showed the visual elements at first then as soon as I click on one, they all disappear.
@emmanuel - any ideas?