Steps to reproduce:
- Click on "edit source" in https://www.wikidata.org/wiki/Wikidata:Tools/Query_data#SPARQL_query_builders,
- Click "Publish changes"
- Click "Show preview"
The save dialog freezes, stuck with the progress animation on top.
I don't think the diff itself makes a difference, but in any case, this was the edit I was doing: