Tmux notify when someone write me on Irssi

I believe this setting is what you're looking for:

/set beep_when_window_active on
/save

So, thanks to Screwtape, I've got an answer for this. Reproduced below:

The original xterm mouse protocol only supports up to 223 columns and lines. Later versions of the protocol used UTF-8 encoding, which turned out to be a terrible idea, and more modern terminals support a completely different protocol based on the SGR escape sequence that's much more sane.

tmux automatically supports the original protocol and the SGR protocol both ways (to the terminal it's running inside of, and to the applications running inside it.) but it only requests UTF-8 mode from the outer terminal if 'mouse-utf8' is enabled in the config file.

You can use the vttest tool to experiment with different mouse protocols inside and outside tmux to get a better idea what's going on.

According to the documentation for Vim's 'ttymouse' option, it will ask the terminal for original-xterm-protocol support if $TERM is a variant of xterm, mlterm or screen (and tmux reports itself as screen, so that's OK). If the terminal supports the "RV" termcap feature to determine the xterm version number, Vim will use that to automatically upgrade to 'xterm2' or 'sgr' mouse protocols... but typically only xterm advertises "RV" support, so tmux is left out of the automatic upgrade process.

In my ~/.vimrc I have the following code to enable decent mouse support, instead of relying on Vim's autodetection:

" Screen/tmux can also handle xterm mousiness, but Vim doesn't
" detect it by default.
if &term == "screen"
set ttymouse=xterm2
endif

if v:version >= 704 && &term =~ "^screen"
" Odds are good that this is a modern tmux, so let's pick the
" best mouse-handling mode.
set ttymouse=sgr
endif

(Of note, the set ttymouse=sgr is the actual fix, here; I suggest adding it to your .vimrc as Tim did.)