Is there a quick way to move a document from the main editor into a copy holder either in the same or the other editor?
Navigate ▸ Open ▸ Open in Copyholder menu command is pretty good for this, especially given you can give it a custom shortcut. I definitely go for that option myself, since I use it a lot.
Otherwise for mouse usage, just click and drag on the icon in the header bar, back onto the header bar, with the Option key held down.
Fantastic! I remember you told me these tricks several years ago, but I never wrote them down. The mouse trick works. Thank you again!