Opens a temporary overlay.
When the preview ends, the overlay will be dismissed
with a dismissal reason of ":undo-preview".
When the user dismisses the temporary overlay before the server responds,
the previewed request will be aborted.
When previewing a render pass that is going to open an overlay,
the temporary overlay will use the same visual style and animations.
Example
This preview opens a temporary overlay with a loading message:
up.preview('spinner-overlay', function(preview) {
preview.openOverlay('Please wait...')
})
