To reproduce:

• Go to https://en.wikipedia.org/wiki/Template:Daat_enc
• Click "Edit source"
• Click "Manage TemplateData"
• Click "Add 6 suggested parameters"
• Click the "X" button in the top left corner

Observed: There is an animation with stripes in the title bar and nothing else happens. The dialog gets stuck.

Expected: The dialog is supposed to be closed. (Maybe it is also supposed to ask whether to save the the changes, but I don't remember.)

I could reproduce it also in the Hebrew Wikipedia.

This happens with any template, not just {{Daat enc}}.

I tested in Firefox 73.

If you click the X in the dialog immediately after opening it and without editing anything, then it works as expected. It only gets stuck after editing.