Was playing with aerc and sixel a bit (Ubuntu 22.04, aerc commit 6b97085a, vaxis 0.11.0, tmux next-3.6 with the default configuration), and noticed that this happens:

• temrinal with sixel support: shows sixel
• temrinal with sixel support -> tmux: shows sixel
• temrinal with sixel support -> aerc: shows sixel
• temrinal with sixel support -> tmux -> aerc: does not show sixel
• temrinal with sixel support -> aerc -> tmux: does not show sixel (shows SIXEL IMAGE (38x15)+++...)

I tested it with this https://github.com/csdvrx/sixel-testsuite/blob/master/snake.six image in aerc's :terminal (so I was able to launch tmux inside aerc, too, the last case :)

Terminals in question were alactritty (via https://github.com/alacritty/alacritty/pull/4763 ), xterm 372 (default for Ubuntu 22.04), xterm 397 and mlterm 3.9.3. The last two, however, provided a bit different results:

• temrinal with sixel support: shows sixel
• temrinal with sixel support -> tmux: shows sixel
• temrinal with sixel support -> aerc: shows sixel, however, the image can't be cleared with <C-l> or clear, and if a TUI program is launched afterwards, sixel image stays on top of it (https://qu.ax/JPuYG.png )
• temrinal with sixel support -> tmux -> aerc: doesn't support true colors, shows sixel with artifacts (https://qu.ax/OoCkE.png ), the image can't be cleared [...]
• temrinal with sixel support -> aerc -> tmux: does not show sixel (shows SIXEL IMAGE (38x15)+++...)

Honestly don't know who's to blame here but since something gets off inside vaxis, thought maybe I'd simply share these results with you :)