What should I do before opening an issue?

Before opening an issue, please ensure that:

Your problem is a specific problem or question or suggestion, not a general complaint.
$TERM inside tmux is screen, screen-256color, tmux or tmux-256color. Check by running echo $TERM inside tmux.
You can reproduce the problem with the latest tmux release, or a build from Git master.
Your question or issue is not covered in the manual (run man tmux).
Your problem is not mentioned in the CHANGES file.
Nobody else has opened the same issue recently.

What should I include in an issue?

Please include the output of:

uname -sp && tmux -V && echo $TERM

Also include:

Your platform (Linux, macOS, or whatever).
A brief description of the problem with steps to reproduce.
A minimal tmux config, if you can't reproduce without a config.
Your terminal, and $TERM inside and outside of tmux.
Logs from tmux (see below). Please attach logs to the issue directly rather than using a download site or pastebin. Put in a zip file if necessary.
At most one or two screenshots, if helpful.

Run a separate tmux server with -f/dev/null to skip loading .tmux.conf:

tmux -Ltest kill-server
tmux -Ltest -f/dev/null new

Add -vv to tmux to create three log files in the current directory. If you can reproduce without a configuration file:

tmux -Ltest kill-server
tmux -vv -Ltest -f/dev/null new

Or if you need your configuration:

tmux kill-server
tmux -vv new

The log files are:

Please attach the log files to your issue.

All it means is that work on the issue is not planned for the near future. See the issue's comments to find out if contributions would be welcome.