TL;DR) tmux passthrough using \033Ptmux; ... \033\ control sequence doesn't work for a very long sequence.

Environment

• Darwin i386 (macOS)
• tmux 2.7 (also confirmed in the latest HEAD as of 10/14)
• screen-256color (also confirmed in xterm-256color etc.)

Description

iTerm2 supports a file download (or image display) protocol. It works perfectly outside tmux, but not working inside tmux well (provided wrapped by tmux passthrough control sequence).

It works outside tmux:

Inside tmux, only small data sequence works. However, large files are not submitted; seems data transmission is interrupted and packets are not all sent.

For a file of 262144 bytes, the data seems to be truncated in the middle.

Please note that this feature is widely used for imgcat, etc. As a result, we cannot display a large image file inside tmux.

How to reproduce?

To reproduce, consider a test script test-iterm.sh, which sends a dummy file of specified number of bytes through iTerm2's file download protocol. Note that when running under tmux, the pass-through control sequence \033Ptmux; ... \033\ is applied on OSC and ST.

#!/bin/bash

function print_osc() {
    if [[ -n $TMUX ]] ; then
        printf "\033Ptmux;\033\033]";
    else printf "\033]"; fi
}

function print_st() {
    if [[ -n $TMUX ]] ; then
        printf "\a\033\\"
    else printf "\a"; fi
}

function send_file() {
    # sends a dummy file containing $REPEAT '.' characters
    REPEAT=${1:-1024}
    filename=$(echo -ne "test-$REPEAT.txt" | base64)
    echo "Sending a text data of $REPEAT bytes ..."

    print_osc
    printf "1337;File=name=$filename;size=$REPEAT;inline=0:"
    head -c $REPEAT < /dev/zero | tr '\0' '.' | base64
    print_st
}

send_file $1

Suspects?

I have no very good idea why it is happening. Any thoughts?

• Seems related to TMUX eats SIXEL DCS header #1388 -- input timer may just flushes data out. Not sure.
• Buffer size limit ('tmux;' passthrough: large sequences #487), which is currently 1M. However, the data being transmitted is less than 1MB (see test-262144.txt example), so I doubt of it.
• Sequence passthrough #846 -- not relevant.

What else did I try?

I tried to split the sequence into multiple chunks (e.g. use ... \033\ + \033Ptmux; ...), but no success.