"tmux attach" does not copy to system clipboard when inside a "tmux popup"

[Spencer Smith]

Hello!

I am using tmux 3.5a on MacOS in Wezterm. I replicated this behavior in iTerm2, default terminal, and Ghostty. 

When I run the command `tmux popup -E "tmux attach"`, the expected session is attached in the popup window. While in the popup window, I enter copy mode, select some text, and hit <enter>. I can see the selected text if I run `:showb`, but the text is not in my system clipboard. 

If I run `tmux popup`, I can repeat the above steps to copy text and the text is in my system clipboard.

If I run `tmux attach` with no sessions attached, it successfully restores my tmux sessions and copies text to the clipboard.

This behavior leads me to believe there is something strange going on when `tmux attach` is run inside of `tmux popop`. Am I simply missing something obvious here?