Screen does not have UTF8 support, tmux does.
Otherwise just a bunch of more sane original defaults in tmux to make things much familiar.
In 2026 if given a choice between screen and tmux to use/learn, most are going to go with tmux.
“‘-U’
Run screen in UTF-8 mode. This option tells screen that your terminal sends and understands UTF-8 encoded characters. It also sets the default encoding for new windows to ‘utf8’.”
— <https://www.gnu.org/software/screen/manual/screen.html#Invok...>
“Command: defutf8 state
(none) Same as the ‘utf8’ command except that the default setting for new windows is changed. Initial setting is on if screen was started with ‘-U’, otherwise off.”
— <https://www.gnu.org/software/screen/manual/screen.html#Chara...>
This can lead to ghost characters that aren't shown, overlapping characters, etc.
It was an attempt to bolt on UTF8 support, but it's not great.