For the client side popup the attribute
collapsedMode="hidden"
must be set.
So the hidden popup is already rendered on the site. Otherwise a client side opened popup has no content.
This leads to the problem of violations of hidden content.
In this case you cannot press the 'Submit'-button if the required field is empty.
Even if the popup is hidden.
The 'Submit'-button in the popup will close the popup, because the server didn't know the current state.
To avoid server request, the attribute
omit="true"
is added to the 'Open'-button and the 'Close'-button.