After a day of "drag to copy doesn't work", we got it working. Here's why.
The problem: you're on your Mac, SSH'd into a server, inside tmux. You drag to select text. You want it on your Mac clipboard.
The four layers: Mac terminal ← SSH ← tmux ← you.
Any one can drop the copy. If you don't understand which layer is responsible for what, you'll stack broken workarounds.