[Bug]: Help char after a prefix key with no binding (e.g. C-c C-h) exits Neomacs #576

Bug summary

Pressing help-char (or any event in help-event-list, such as ? or <f1>) after a prefix key closes Neomacs when that combination has no binding of its own. describe-prefix-bindings runs and prepares the *Help* buffer, then the command loop exits and the process ends with status 0. Nothing is reported as an error; the log only says "Command loop exited normally" and "Neomacs exited cleanly".

Combinations that have a real binding are not affected, because they never reach the prefix-help path: C-h C-h (help-for-help), or C-c C-h after (global-set-key (kbd "C-c C-h") #'forward-char).

It happens with -Q, both in the GUI (X11) and with -nw.

Steps to reproduce

1. Run neomacs -Q -nw (the GUI behaves the same).
2. In *scratch*, type C-c C-h.
3. "Type C-x 1 to delete the help window." is logged, then Neomacs exits with status 0.

C-x 4 C-h and C-c ? do the same.

Expected behavior

Same as GNU Emacs: a *Help* window lists the bindings that start with C-c, and the session continues.

Neomacs --version output

Neomacs 0.0.19
Git commit: unknown
Source date: unknown
Build: release/bench for x86_64-unknown-linux-gnu with rustc 1.96.1
Standalone Rust binary for Neomacs (no C dependency)

Platform

Linux (X11)

Install source

Built from source, some other way

Extra context

Cause, as far as I can tell

dispatch_prefix_help_command calls prefix-help-command itself, then returns an empty key sequence with a nil binding, expecting command_loop_1 to treat that as "nothing to dispatch":

read_command_key_sequence_with_options treats exactly that result as end of input:

so command_loop_1 returns:

GNU does not run the help command inside read_key_sequence. It records prefix-help-command as the command for the sequence and returns the whole sequence, help char included:
https://github.com/emacs-mirror/emacs/blob/8f535370b9efbc91673b20c6987a5cae4f6dc562/src/keyboard.c#L11324-L11329

command_loop_1 then treats only a zero-length sequence as end of file, and runs the recorded command like any other, with pre-command-hook / post-command-hook and this-command:
https://github.com/emacs-mirror/emacs/blob/8f535370b9efbc91673b20c6987a5cae4f6dc562/src/keyboard.c#L1439-L1443
https://github.com/emacs-mirror/emacs/blob/8f535370b9efbc91673b20c6987a5cae4f6dc562/src/keyboard.c#L1469

describe-prefix-bindings drops the last event of (this-command-keys) itself. Returning the current sequence with prefix-help-command as its binding, instead of calling it and returning an empty sequence, would match GNU.

Debug log excerpt

RUST_LOG=debug NEOMACS_LOG=info NEOMACS_LOG_FILE=... neomacs -nw -Q, then C-c C-h. Span prefixes and timestamps are trimmed.

DEBUG neomacs_display_runtime::tty_input: tty_input: raw byte batch RawTtyBytes { bytes: [3], emacs_frame_id: 0 }
DEBUG neovm_core::keyboard: read_key_sequence: event=3 starting translation
DEBUG neovm_core::keyboard: read_key_sequence: looking up binding for ["3"]
DEBUG neovm_core::keyboard: read_key_sequence: binding=(keymap (2 . elisp-byte-compile-buffer) (5 . elisp-eval-region-or-buffer))
DEBUG neomacs_display_runtime::tty_input: tty_input: raw byte batch RawTtyBytes { bytes: [8], emacs_frame_id: 0 }
DEBUG neovm_core::keyboard: read_key_sequence: event=8 starting translation
DEBUG neovm_core::keyboard: read_key_sequence: looking up binding for ["3", "8"]
DEBUG neovm_core::keyboard: read_key_sequence: binding=nil
 INFO neovm_core::emacs_core::load: load .../share/neomacs/lisp/help-fns.elc
 ...
 INFO neovm_core::emacs_core::builtins::misc_pure: msg=Type C-x 1 to delete the help window.
DEBUG neovm_core::emacs_core::eval::command_loop: command_loop_inner: result=true, propagating
 INFO neomacs: Command loop exited normally
 INFO neomacs: Shutting down...
 INFO neomacs::tty_init: TTY terminal restored
 INFO neomacs: Neomacs exited cleanly duration_ms=5614 command_args=[..., "-nw", "-Q"]

A Lisp-side workaround that avoids the exit: (setq prefix-help-command nil), plus an advice on undefined that calls describe-prefix-bindings when the last event is a help event.