shift modifiers don't work in terminal multiplexers with a similar non-shift modifier key binding

[ifiokjr]

Summary

With the following code in my config.toml I have two similar commands C-j and C-J. In a normal terminal, these work fine. However, when I use a terminal multiplexer like tmux or zellij only the C-j is recognised.

[keys.normal]
## Move selection up
C-k = "@X\"mdk\"mP"
## Move selection down
C-j = "@X\"mdj\"mP"
## Copy the current line / selection to the line above.
C-K = "@X\"my\"mP"
## Copy the current line / selection to the line below.
C-J = "@X\"my\"mp"

In normal mode inside zellij or tmux I don't have access to C-J or C-K which copy the current line / selection down or up. These key mappings work in the bare terminal (I use ghostty).

Reproduction Steps

Before all these steps: install zellij or tmux then activate in the terminal. Add these keybindings to the helix config.toml.

[keys.normal]
## Move selection up
C-k = "@X\"mdk\"mP"
## Move selection down
C-j = "@X\"mdj\"mP"
## Copy the current line / selection to the line above.
C-K = "@X\"my\"mP"
## Copy the current line / selection to the line below.
C-J = "@X\"my\"mp"

1. hx
2. Paste this text into an empty buffer

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comes from a line in section 1.10.32. 

3. Move to the 2nd line.
4. Try C-J which should copy the current line above.

I expected this to happen:

The line should be copied above in a terminal multiplexer environment.

Instead, this happened:

It ignores the <shift> modifier and acts like you've pressed C-j.

Helix log

No response

Platform

macOS

Terminal Emulator

ghostty

Installation Method

brew

Helix Version

25.01.1

[the-mikedavis]

To recognize C-J you need to use a terminal with the Kitty keyboard protocol enabled, like Ghostty. That protocol emits escape codes for keys which can otherwise be ambiguous. For another example C-h is equivalent to backspace. I'm pretty sure that tmux doesn't support the Kitty keyboard protocol. I know that Zellij has some support for it but not all of the parts of the protocol that we use.

[ifiokjr]

From this discussion thread: zellij-org/zellij#3789 my understanding is as follows.

imsnif (the core maintainer) points out that zellij correctly implements the first level of the Kitty Keyboard Protocol and supports Shift modifiers. However, helix, through crossterm uses a boolean supported: true or supported: false for the Kitty Keyboard Protocol, without accounting for partial implementations.

Is the solution to support 100% of the kitty protocol in zellij or to add support for partial implementations to crossterm?

[the-mikedavis]

Supporting both of the parts of the protocol we use in Zellij would be nice from the Helix perspective. It sounds like adding that to Zellij is not desirable though. On the Helix side we need some changes to the terminal backend (currently crossterm) to query for what parts of the protocol the terminal provides. If it doesn't support both of the parts that we need then we would disable the protocol so regular keyboard handling would happen instead. (Meaning C-J and C-h would behave in their default odd way.) This is currently done in #13307