Support bindings with the Super modifier

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.

book/src/remapping.md

@@ -51,7 +51,10 @@ t = ":run-shell-command cargo test"
 ## 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:
+`C-`, `S-` and `A-`. The Super modifier (the Windows/Linux key or the Command
+key on Mac keyboards) is also supported for terminals which support the enhanced
+keyboard protocol (Kitty, WezTerm). The Super key uses the `Super-` prefix.
+Special keys are encoded as follows:
 
 | Key name     | Representation |
 | ---          | ---            |

helix-view/src/input.rs

@@ -158,7 +158,12 @@ 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) {
+                "Super-"
+            } else {
+                ""
+            },
             if self.modifiers.contains(KeyModifiers::SHIFT) {
                 "S-"
             } else {
@@ -308,6 +313,9 @@ impl UnicodeWidthStr for KeyEvent {
         if self.modifiers.contains(KeyModifiers::CONTROL) {
             width += 2;
         }
+        if self.modifiers.contains(KeyModifiers::SUPER) {
+            width += 6;
+        }
         width
     }
 
@@ -392,6 +400,7 @@ impl std::str::FromStr for KeyEvent {
                 "S" => KeyModifiers::SHIFT,
                 "A" => KeyModifiers::ALT,
                 "C" => KeyModifiers::CONTROL,
+                "Super" => KeyModifiers::SUPER,
                 _ => return Err(anyhow!("Invalid key modifier '{}-'", token)),
             };
 
@@ -678,6 +687,14 @@ mod test {
                 modifiers: KeyModifiers::ALT | KeyModifiers::CONTROL
             }
         );
+
+        assert_eq!(
+            str::parse::<KeyEvent>("Super-c").unwrap(),
+            KeyEvent {
+                code: KeyCode::Char('c'),
+                modifiers: KeyModifiers::SUPER
+            }
+        );
     }
 
     #[test]

helix-view/src/keyboard.rs

@@ -7,6 +7,7 @@ bitflags! {
         const SHIFT = 0b0000_0001;
         const CONTROL = 0b0000_0010;
         const ALT = 0b0000_0100;
+        const SUPER = 0b0000_1000;
         const NONE = 0b0000_0000;
     }
 }
@@ -27,6 +28,9 @@ impl From<KeyModifiers> for crossterm::event::KeyModifiers {
         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 @@ impl From<crossterm::event::KeyModifiers> for KeyModifiers {
         if val.contains(CKeyModifiers::ALT) {
             result.insert(KeyModifiers::ALT);
         }
+        if val.contains(CKeyModifiers::SUPER) {
+            result.insert(KeyModifiers::SUPER);
+        }
 
         result
     }