Would you be able to take a look at a potential bug when the Require Checkout feature is enabled.
When a user edits and item that is not currently checked out they are unable to cancel the form and close the dialog.
The window checkin/out prompts loop without closing the dialog.
1. Navigate to /sites/dffs/Lists/CheckInOutBug/AllItems.aspx
2. Ensure the item is currently not checked out.
3. Edit the item
4. Click “OK” to checkout the item.
5. Click “Cancel”
6. Click “OK”
7. “Repeat steps 4-5” and the form can never be cancelled.