This maybe a simple universal thing but how do I add a document to an existing window without using “merge all windows”. Ive tried dragging it but it doesn’t work. Maybe its not possible?
You need to open the document into its own window (so you have both the original window and new document window open at once) and then use “Merge All Windows”.
The latest preview release changes things around a bit:
You can now use Option-Command-N to add a new document as a tab. To open a new view on an existing document to use Window > Duplicate Tab