"Invalid active_pane_id received", connection closed closing a pane

[inducer]

As of 1f31412, the following set of steps (using tmate 2.4.0 from Debian) lead to the connection being dropped by the server, and a "Reconnecting..." message appearing:

• Run tmate configured against a custom server running 1f31412.
• Ctrl-b c to create a new terminal
• Ctrl-b c to create another new terminal (not sure this is essential)
• Ctrl-d to close the terminal
• Boom, connection dropped, "Reconnecting..." appears, and the session is dead for all I can tell.

On the server, a message like the following is printed:

Mar 07 16:43:30 meet tmate-ssh-server[323592]: [wWWG...] fatal: Invalid active_pane_id recevied

Simply dropping/reverting 1f31412 appears to address the issue.

[Zoooook]

Everybody on my team has this issue, on various flavors of Ubuntu and macOS.