tmux: enable sixel support

[laggardkernel]

Description

tmux added SIXEL support since 3.4, enable this feature during build.

https://github.com/tmux/tmux/blob/2df15ad08c9e4cf286cdb38cb645fc9066c3098d/CHANGES#L102

test method, build and open shell in tmux, install ImageMagick

convert /path/to/pic sixel:-

You should get a preview of the picture within tmux, if your terminal also support SIXEL images. (terminals including iTerm2, wezterm, etc)

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 12.7 21G816 arm64
Command Line Tools 14.2.0.0.1.1668646533

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint --nitpick?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install? (without -t trace mode, which is broken)
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@tessus for port tmux.

[casr]

What would be the result with terminals that do not support Sixel such as Terminal.app or Alacritty?

Should it be applied as a variant instead?

edit: And if the first point has no issues, should this be enabled as a default variant?

[laggardkernel]

@casr SIXEL is a protocol defines how to convert image file into escape sequences. ImageMagick is a tool who supports this process.

For terminals who understand this protocol, it will render the generated escape sequence as picture and display it in the terminal. For those who don't, in my test, they just ignore it and do nothing.

Should it be applied as a variant instead?

I don't think so. It works only when user try to preview an image in the terminal explicitly, using some tools like ImageMagick.

Tmux doesn't render the image directly, it works as a middleman in the preview process. The sixel support of tmux just make sure the escape sequence is passed out to terminals, untouched, but not intercept the sequence and handle it by tmux itself.

[tessus]

I agree, this is the same as other escape sequences being sent by tmux natively. If the terminal does not understand them, it depends on the terminal to handle this situation.

TL;DR
Terminals are notorious for ignoring standards and/or extending the functionality without proper specs. In a perfect world, apps could query the terminal whether it supports certain features and in many cases this even works. However, it is not something an app can rely on completely. Thus there are many workarounds in a lot of apps to support multiple terminals seemlessly.

[casr]

I'll argue for a variant because:

1. tmux treats this as a feature that must be enabled
2. it's suffered resistance in the past with some projects adopting it due to things like performance concerns or 'why this standard over another?'

For those reasons, I think having the ability to toggle this as a variant would be welcomed by some.

I do not see a problem making this a default variant though.