key: Actually do engage legacy mode if first char is control

This was already in the comment.

Fixes #10450

CHANGELOG.rst

@@ -40,7 +40,7 @@ Notable backwards-incompatible changes
       Since ``,`` and ``-`` act as separators, there are some cases where they need to be written as ``comma`` and ``minus`` respectively.
   - To minimize gratuitous breakage, the key argument to ``bind`` is parsed using the old syntax in two cases:
 
-    - If the key argument starts with a raw escape character (``\e``). Besides backwards compatibility, these "raw" bindings can be useful for keys that fish can't decode yet.
+    - If the key argument starts with a raw escape character (``\e``) or ascii control character. Besides backwards compatibility, these "raw" bindings can be useful for keys that fish can't decode yet.
     - If the key argument consists of exactly two characters, contains none of ``,`` or ``-`` and is not a named key.
 
 - Fish no longer supports terminals that fail to ignore OSC or CSI sequences they don't recognize.

src/key.rs

@@ -238,7 +238,7 @@ pub(crate) fn parse_keys(value: &wstr) -> Result<Vec<Key>, WString> {
         && !value.contains(KEY_SEPARATOR)
         && !KEY_NAMES.iter().any(|(_codepoint, name)| name == value)
         && value.as_char_slice()[0] != 'F')
-        || first == '\x1b'
+        || first == '\x1b' || first < ' '
     {
         // Hack: treat as legacy syntax (meaning: not comma separated) if
         // 1. it doesn't contain '-' or ',' and is short enough to probably not be a key name.