Support bindings with the Super (Cmd/Win/Meta) modifier (#6592)

Terminals which support the enhanced keyboard protocol send events for
keys pressed with the Super modifier (Windows/Linux key or the Command
key). The only changes that are needed to support this in Helix are:

* Mapping the modifier from crossterm's KeyModifiers to Helix's
  KeyModifiers.
* Representing and parsing the modifier from the KeyEvent text
  representation.
* Documenting the ability to remap it.

When writing keybindings, use 'Meta-', 'Cmd-' or 'Win-' which are all
synonymous. For example:

    [keys.normal]
    Cmd-s = ":write"

will trigger for the Windows or Linux keys and the Command key plus 's'.

book/src/remapping.md

@@ -72,8 +72,22 @@ # create a new minor mode bound to `+`
 
 ## Special keys and modifiers
 
-Ctrl, Shift and Alt modifiers are encoded respectively with the prefixes
-`C-`, `S-` and `A-`. Special keys are encoded as follows:
+Ctrl, Shift and Alt modifiers are encoded respectively with the prefixes `C-`, `S-` and `A-`.
+
+The [Super key](https://en.wikipedia.org/wiki/Super_key_(keyboard_button)) - the Windows/Linux
+key or the Command key on Mac keyboards - is also supported when using a terminal emulator that
+supports the [enhanced keyboard protocol](https://github.com/helix-editor/helix/wiki/Terminal-Support#enhanced-keyboard-protocol).
+The super key is encoded with prefixes `Meta-`, `Cmd-` or `Win-`. These are all synonyms for the
+super modifier - binding a key with a `Win-` modifier will mean it can be used with the
+Windows/Linux key or the Command key.
+
+```toml
+[keys.normal]
+C-s = ":write" # Ctrl and 's' to write
+Cmd-s = ":write" # Cmd or Win or Meta and 's' to write
+```
+
+Special keys are encoded as follows:
 
 | Key name     | Representation |
 | ---          | ---            |

helix-view/src/input.rs

@@ -162,7 +162,12 @@ pub(crate) mod keys {
 impl fmt::Display for KeyEvent {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> std::fmt::Result {
         f.write_fmt(format_args!(
-            "{}{}{}",
+            "{}{}{}{}",
+            if self.modifiers.contains(KeyModifiers::SUPER) {
+                "Meta-"
+            } else {
+                ""
+            },
             if self.modifiers.contains(KeyModifiers::SHIFT) {
                 "S-"
             } else {
@@ -312,6 +317,10 @@ fn width(&self) -> usize {
         if self.modifiers.contains(KeyModifiers::CONTROL) {
             width += 2;
         }
+        if self.modifiers.contains(KeyModifiers::SUPER) {
+            // "-Meta"
+            width += 5;
+        }
         width
     }
 
@@ -413,6 +422,7 @@ fn from_str(s: &str) -> Result<Self, Self::Err> {
                 "S" => KeyModifiers::SHIFT,
                 "A" => KeyModifiers::ALT,
                 "C" => KeyModifiers::CONTROL,
+                "Meta" | "Cmd" | "Win" => KeyModifiers::SUPER,
                 _ => return Err(anyhow!("Invalid key modifier '{}-'", token)),
             };
 
@@ -733,6 +743,28 @@ fn parsing_modified_keys() {
                 modifiers: KeyModifiers::NONE
             }
         );
+
+        assert_eq!(
+            str::parse::<KeyEvent>("Meta-c").unwrap(),
+            KeyEvent {
+                code: KeyCode::Char('c'),
+                modifiers: KeyModifiers::SUPER
+            }
+        );
+        assert_eq!(
+            str::parse::<KeyEvent>("Win-s").unwrap(),
+            KeyEvent {
+                code: KeyCode::Char('s'),
+                modifiers: KeyModifiers::SUPER
+            }
+        );
+        assert_eq!(
+            str::parse::<KeyEvent>("Cmd-d").unwrap(),
+            KeyEvent {
+                code: KeyCode::Char('d'),
+                modifiers: KeyModifiers::SUPER
+            }
+        );
     }
 
     #[test]

helix-view/src/keyboard.rs

@@ -7,6 +7,7 @@ pub struct KeyModifiers: u8 {
         const SHIFT = 0b0000_0001;
         const CONTROL = 0b0000_0010;
         const ALT = 0b0000_0100;
+        const SUPER = 0b0000_1000;
         const NONE = 0b0000_0000;
     }
 }
@@ -27,6 +28,9 @@ fn from(key_modifiers: KeyModifiers) -> Self {
         if key_modifiers.contains(KeyModifiers::ALT) {
             result.insert(CKeyModifiers::ALT);
         }
+        if key_modifiers.contains(KeyModifiers::SUPER) {
+            result.insert(CKeyModifiers::SUPER);
+        }
 
         result
     }
@@ -48,6 +52,9 @@ fn from(val: crossterm::event::KeyModifiers) -> Self {
         if val.contains(CKeyModifiers::ALT) {
             result.insert(KeyModifiers::ALT);
         }
+        if val.contains(CKeyModifiers::SUPER) {
+            result.insert(KeyModifiers::SUPER);
+        }
 
         result
     }