daemon: legacy key encoding drops Ctrl from Space and Backspace and the case after Alt #157

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opened 2026-10-18 12:41:50 +09:00 by natsukium · 0 comments

natsukium commented

2026-10-18 12:41:50 +09:00

Owner

Reported on the GitHub mirror: https://github.com/felis-terminal/felis/issues/5

Problem

With no Kitty keyboard flag that turns on CSI u and modifyOtherKeys off, the legacy path in crates/felis-daemon/src/serve/key_encode.rs encode sends some modified keys differently from xterm and kitty:

Key	felis	xterm / kitty
Ctrl+Space	`0x20`	`0x00`
Ctrl+Backspace	`0x7f`	`0x08`
Alt+Shift+B (`Key::Character("B")`)	`ESC b`	`ESC B`
Ctrl+2 … Ctrl+8, Ctrl+/, Ctrl+?	the plain text, or nothing	`0x00`, `0x1b`, `0x1c`, `0x1d`, `0x1e`, `0x1f`, `0x7f`; `/` → `0x1f`, `?` → `0x7f`

This is the GUI's own path, so felis users hit it: Emacs C-SPC (set-mark), readline M-B vs M-b, and a shell's Ctrl+Backspace word delete all receive a different key.

Cause

encode_named returns the bare byte for Space and Backspace without looking at Ctrl, and returns Some, so the Ctrl branch after it never runs.
The metaSendsEscape branch builds its body with single_ascii_lower, which exists for the Kitty keycode ("the lowercase ASCII codepoint of the key") and drops the case Shift produced.
ctrl_byte maps letters and @ [ \ ] ^ _ but no digits, / or ?.

Expected

The legacy path matches xterm for these, as it does for the rest:

Ctrl+Space → 0x00, Ctrl+Backspace → 0x08; Alt prefixes ESC as it does for the other named keys.
Alt+character keeps the character's case.
Ctrl+digit / punctuation follow xterm's table: 2 / @ / Space → 0x00, 3 / [ → 0x1b, 4 / \ → 0x1c, 5 / ] → 0x1d, 6 / ^ → 0x1e, 7 / _ / / → 0x1f, 8 / ? → 0x7f.

The modifyOtherKeys and Kitty paths already escape these keys and are unaffected. The legacy section of docs/reference/protocols/key-encoding.md should name the mapping once it is fixed.

Reported on the GitHub mirror: https://github.com/felis-terminal/felis/issues/5 ## Problem With no Kitty keyboard flag that turns on CSI u and modifyOtherKeys off, the legacy path in `crates/felis-daemon/src/serve/key_encode.rs` `encode` sends some modified keys differently from xterm and kitty: | Key | felis | xterm / kitty | | --- | --- | --- | | Ctrl+Space | `0x20` | `0x00` | | Ctrl+Backspace | `0x7f` | `0x08` | | Alt+Shift+B (`Key::Character("B")`) | `ESC b` | `ESC B` | | Ctrl+2 … Ctrl+8, Ctrl+/, Ctrl+? | the plain text, or nothing | `0x00`, `0x1b`, `0x1c`, `0x1d`, `0x1e`, `0x1f`, `0x7f`; `/` → `0x1f`, `?` → `0x7f` | This is the GUI's own path, so felis users hit it: Emacs `C-SPC` (set-mark), readline `M-B` vs `M-b`, and a shell's Ctrl+Backspace word delete all receive a different key. ## Cause - `encode_named` returns the bare byte for `Space` and `Backspace` without looking at Ctrl, and returns `Some`, so the Ctrl branch after it never runs. - The metaSendsEscape branch builds its body with `single_ascii_lower`, which exists for the Kitty keycode ("the lowercase ASCII codepoint of the key") and drops the case Shift produced. - `ctrl_byte` maps letters and `@ [ \ ] ^ _` but no digits, `/` or `?`. ## Expected The legacy path matches xterm for these, as it does for the rest: - Ctrl+Space → `0x00`, Ctrl+Backspace → `0x08`; Alt prefixes `ESC` as it does for the other named keys. - Alt+character keeps the character's case. - Ctrl+digit / punctuation follow xterm's table: `2` / `@` / Space → `0x00`, `3` / `[` → `0x1b`, `4` / `\` → `0x1c`, `5` / `]` → `0x1d`, `6` / `^` → `0x1e`, `7` / `_` / `/` → `0x1f`, `8` / `?` → `0x7f`. The modifyOtherKeys and Kitty paths already escape these keys and are unaffected. The legacy section of `docs/reference/protocols/key-encoding.md` should name the mapping once it is fixed.

natsukium added the

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2026-10-18 12:41:54 +09:00

natsukium referenced this issue from a pull request that will close it,

2026-10-18 13:14:36 +09:00

daemon, docs: send xterm's legacy bytes for Ctrl+Space, Ctrl+digit and Alt+Shift #163

natsukium closed this issue

2026-10-18 13:44:06 +09:00