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Quote from solution, posted on Github, in the issue mentioned by @PabloBianchi:
Apparently, to make byobu and powerline correctly display the window names, you will need to add the default command setting to your .tmux.conf (in my case fish):
set-option -g default-command "/usr/bin/fish"
Notably, only the first window will get the name, and the rest will get only a weird hyphen (-) if you create the window using byobu's F2.
However, if you use the standard -c keybinding for the window creation, then all windows will have proper names.
Might be related to this issue.
– Pablo A – 2017-09-17T02:43:58.317@PabloBianchi Thank you! – spellanser – 2019-06-17T15:43:46.680