Support extended keys

[mistotebe]

Several terminals support the Extended keyboard protocol which emits keys in a way that's much easier to parse. It has to be requested, but would allow textual to receive any key+modifier combination.

xterm also sends a (seemingly compatible) subset, which the specification is based on. In the longer term, this might enable keypress/release monitoring and other features, AFAIK not necessary to handle right now.

See also https://github.com/tmux/tmux/wiki/Modifier-Keys

[willmcgugan]

This is definitely worth looking in to. @darrenburns @davep

[davep]

Seconded. If it carries on working fine on the least-best-supported emulators but improves things in better-supported ones I really like the idea.

[darrenburns]

The protocol has a specific escape sequence for Esc so you don't need craziness to differentiate between presses of the escape key and the start of an escape sequence - wow.

(While I do think this is cool, the lack of support in Apple Terminal, Windows Terminal, and iTerm2 make me wonder how we can nicely integrate this in a way that makes it clear that these are only supported in some environments)

[willmcgugan]

The Extended key protocol is now supported in Textual.

[github-actions]

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