As of Spring 20 release, we can again prepopulate field values using a URL - aka URL hacking (see release note here (https://releasenotes.docs.salesforce.com/en-us/spring20/release-notes/rn_general_lex_navigate_to_record_dfv.htm)).

I, for one, am so happy for this feature. However, there is one minor problem. When the URL is invoked, the new record modal pops up, and if the user chooses to Cancel, a blank page is displayed instead of returning to the page where the URL was called.

Below is the URL that I'm using. It is invoked with a custom list button located in a related list of an Opportunity.

/lightning/o/Case/new?defaultFieldValues=
AccountId={!Opportunity.AccountId},
Opportunity__c={!Opportunity.Id}

I've tried including a retunURL parameter - in different places in the URL. But no luck.