TUI: handle ModifyOtherKeys key sequences

[jerome-pouiller]

Environment

• nvim --version: v0.4.4
• vim -u DEFAULTS (version: ) behaves differently? no
• Operating system/version: Debian Bullseye
• Terminal name/version: xterm
• $TERM: xterm

Steps to reproduce

• Modify your ~/.Xresources to include xterm*modifyOtherKeys: 2. This step is not mandatory, but it may make the further behavior easier to understand. Don't forget to merge your resource with xrdb -merge ~/.Xresources.
• Launch nvim -u NORC with xterm.
• Enter command mode with :.
• Hit C-k twice (I have never understood why C-k should be typed twice, but I think it is not related to the problem).
• Hit a key sequence with Ctrl and Shift like C-S-g.
• It shows ^G while <C-S-g> is expected.

If you do the same with vim, it displays <C-S-g> as expected.

Neovim correctly support ModifyOtherKeys. If it was not the case, it would be impossible to enter any control sequences or shited letters (compare with the behavior of a program that does not support ModifyOtherKeys like nano). However, a part of the escape sequence is dropped somewhere and neovim does not make the difference between <C-g> and <C-S-g>.

Note I think this issue is not related to #6279. Indeed, in my setup xterm send ^]] and not \x9b.

[glacambre]

Could you try nightly? I think this bug might have been fixed already.

[jerome-pouiller]

Indeed, it is fixed with v0.5.0-dev+1233-g82ac44d01

[jerome-pouiller]

In fact, it seems there is another problem with nightly build. Indeed, it runs fine with xterm*modifyOtherKeys: 2 (which is unsane). However, it still does not work with the default xterm configuration. I think that neovim does not send the string \<Esc>[>4;2m that enable modifyOtherKeys[1]. In vim, this string is sent by t_TI and t_TE[2]. Indeed, I do not see these equivalents in neovim.

[1]: as explain here alacritty/alacritty#3101
[2]: see :h modifyOtherKeys in vim

[neovim-discourse]

This issue has been mentioned on Neovim Discourse. There might be relevant details there:

https://neovim.discourse.group/t/nitty-gritty-questions-on-terminal-nvim-and-keyboard-events/1624/1

[justinmk]

fixed by #17771