fix: suppress Windows dead-key fallback characters in remote panes (#3972)

refs #3948

Co-authored-by: akbash-bot <300245827+akbash-bot@users.noreply.github.com>

4 files changed, 154 insertions, 10 deletions

diff --git a/src/client/input/windows_vti.rs b/src/client/input/windows_vti.rs
index d89a02c7..679d5b31 100644
--- a/src/client/input/windows_vti.rs
+++ b/src/client/input/windows_vti.rs
@@ -2546,6 +2546,31 @@ mod tests {
     }
 
     #[test]
+    fn vti_altgr_dead_key_preserves_native_record_without_command_modifiers() {
+        // AltGr+4 press captured in #3948, Spanish ISO layout.
+        let record = WindowsKeyRecord {
+            key_down: true,
+            repeat_count: 1,
+            virtual_key_code: 52,
+            virtual_scan_code: 5,
+            unicode: 0,
+            control_key_state: 9,
+        };
+        let events = translate_with_provenance(win32_input_mode_encoded_record(record));
+        assert_eq!(
+            events,
+            vec![crate::protocol::ClientInputEvent::Key {
+                code: crate::protocol::ClientKeyCode::Char('4'),
+                modifiers: 0,
+                kind: crate::protocol::ClientKeyKind::Press,
+                repeat_count: 1,
+                generated_text: None,
+                source: crate::protocol::ClientKeySource::WindowsConsole { record },
+            }]
+        );
+    }
+
+    #[test]
     fn vti_us_international_dead_key_only_emits_composed_text() {
         fn encode_for_kitty(events: Vec<crate::protocol::ClientInputEvent>, flags: u16) -> Vec<u8> {
             events
diff --git a/src/input/encode.rs b/src/input/encode.rs
index 1dd56351..27e59dce 100644
--- a/src/input/encode.rs
+++ b/src/input/encode.rs
@@ -16,10 +16,10 @@ pub fn encode_key(key: KeyEvent, protocol: KeyboardProtocol) -> Vec<u8> {
 }
 
 pub fn encode_terminal_key(key: TerminalKey, protocol: KeyboardProtocol) -> Vec<u8> {
-    // A zero Unicode value on this Windows character event means the host layout is
-    // still composing a dead key. Kitty panes must not receive its physical fallback.
+    // The host layout has not committed text for this Windows dead key. Neither
+    // legacy nor Kitty panes should receive its physical character fallback.
     // Legacy Windows panes take the native ConPTY fallback before reaching this encoder.
-    if matches!(protocol, KeyboardProtocol::Kitty { .. }) && key.is_windows_shift_dead_key() {
+    if key.is_windows_dead_key() {
         return Vec::new();
     }
 
@@ -572,6 +572,36 @@ mod tests {
     }
 
     #[test]
+    fn kitty_all_keys_does_not_encode_windows_altgr_dead_key_phases() {
+        use crossterm::event::KeyEventKind;
+
+        let key = TerminalKey::new(KeyCode::Char('4'), KeyModifiers::empty()).with_windows_record(
+            crate::input::WindowsKeyRecord {
+                key_down: true,
+                repeat_count: 1,
+                virtual_key_code: 52,
+                virtual_scan_code: 5,
+                unicode: 0,
+                control_key_state: 9,
+            },
+        );
+        for kind in [
+            KeyEventKind::Press,
+            KeyEventKind::Repeat,
+            KeyEventKind::Release,
+        ] {
+            assert!(
+                encode_terminal_key(
+                    key.clone().with_kind(kind),
+                    KeyboardProtocol::Kitty { flags: 31 },
+                )
+                .is_empty(),
+                "{kind:?}"
+            );
+        }
+    }
+
+    #[test]
     fn legacy_enter() {
         let key = KeyEvent::new(KeyCode::Enter, KeyModifiers::empty());
         assert_eq!(encode_key(key, KeyboardProtocol::Legacy), vec![b'\r']);
diff --git a/src/input/model.rs b/src/input/model.rs
index 50cd7e42..3828174f 100644
--- a/src/input/model.rs
+++ b/src/input/model.rs
@@ -78,7 +78,7 @@ pub struct TerminalKey {
     pub shifted_codepoint: Option<u32>,
     pub generated_text: Option<String>,
     physical_identity_hint: bool,
-    windows_shift_dead_key: bool,
+    windows_dead_key: bool,
     source: KeySource,
 }
 
@@ -92,7 +92,7 @@ impl TerminalKey {
             shifted_codepoint: None,
             generated_text: None,
             physical_identity_hint: false,
-            windows_shift_dead_key: false,
+            windows_dead_key: false,
             source: KeySource::Synthesized,
         }
     }
@@ -160,8 +160,10 @@ impl TerminalKey {
         mut self,
         record: Option<WindowsKeyRecord>,
     ) -> Self {
-        self.windows_shift_dead_key = matches!(self.code, KeyCode::Char(_))
-            && self.modifiers == KeyModifiers::SHIFT
+        // AltGr is normalized to text-only modifiers by the Windows input mapper.
+        // Command chords can also have zero Unicode, so retain their fallback keys.
+        self.windows_dead_key = matches!(self.code, KeyCode::Char(_))
+            && self.modifiers.difference(KeyModifiers::SHIFT).is_empty()
             && record.is_some_and(|record| record.unicode == 0);
         self
     }
@@ -189,8 +191,8 @@ impl TerminalKey {
         None
     }
 
-    pub(crate) fn is_windows_shift_dead_key(&self) -> bool {
-        self.windows_shift_dead_key
+    pub(crate) fn is_windows_dead_key(&self) -> bool {
+        self.windows_dead_key
     }
 
     pub(crate) fn identity(&self) -> KeyIdentity {
@@ -423,6 +425,46 @@ mod tests {
     }
 
     #[test]
+    fn windows_composition_hint_requires_uncommitted_text_not_a_command() {
+        let record = WindowsKeyRecord {
+            key_down: true,
+            repeat_count: 1,
+            virtual_key_code: 52,
+            virtual_scan_code: 5,
+            unicode: 0,
+            control_key_state: 9,
+        };
+        for modifiers in [
+            KeyModifiers::CONTROL,
+            KeyModifiers::ALT,
+            KeyModifiers::CONTROL | KeyModifiers::ALT,
+            KeyModifiers::SUPER,
+        ] {
+            let key = TerminalKey::new(KeyCode::Char('4'), modifiers)
+                .with_windows_composition_hint(Some(record));
+            assert!(
+                !key.is_windows_dead_key(),
+                "command modifiers: {modifiers:?}"
+            );
+        }
+        for (code, source) in [
+            (KeyCode::Left, Some(record)),
+            (KeyCode::Char('4'), None),
+            (
+                KeyCode::Char('~'),
+                Some(WindowsKeyRecord {
+                    unicode: 126,
+                    ..record
+                }),
+            ),
+        ] {
+            let key =
+                TerminalKey::new(code, KeyModifiers::empty()).with_windows_composition_hint(source);
+            assert!(!key.is_windows_dead_key(), "{code:?}, {source:?}");
+        }
+    }
+
+    #[test]
     fn release_clears_generated_text_and_grouped_repeat_count() {
         let release = TerminalKey::new(KeyCode::Char('a'), KeyModifiers::empty())
             .with_generated_text(Some("a".to_owned()))
diff --git a/src/protocol/wire.rs b/src/protocol/wire.rs
index 96881b14..052c73a3 100644
--- a/src/protocol/wire.rs
+++ b/src/protocol/wire.rs
@@ -2066,7 +2066,7 @@ mod tests {
         else {
             panic!("pane dead key should remain a key");
         };
-        assert!(key.is_windows_shift_dead_key());
+        assert!(key.is_windows_dead_key());
         assert_eq!(key.windows_record(), None);
         assert!(crate::input::encode_terminal_key(
             key,
@@ -2075,6 +2075,53 @@ mod tests {
         .is_empty());
     }
 
+    #[tokio::test]
+    async fn client_shell_remote_altgr_dead_key_emits_only_composed_text() {
+        // Spanish ISO AltGr+4, then Space, captured in #3948:
+        // https://github.com/herdrdev/herdr/issues/3948#issuecomment-5633222390
+        let records = [
+            ('4', ClientKeyKind::Press, 52, 5, 0, 9),
+            ('4', ClientKeyKind::Release, 52, 5, 0, 9),
+            ('~', ClientKeyKind::Press, 32, 57, 126, 0),
+            (' ', ClientKeyKind::Release, 32, 57, 32, 0),
+        ];
+        let (runtime, _rx) = crate::terminal::TerminalRuntime::test_with_channel(80, 24);
+        let mut output = Vec::new();
+        for (ch, kind, virtual_key_code, virtual_scan_code, unicode, control_key_state) in records {
+            let event = ClientInputEvent::Key {
+                code: ClientKeyCode::Char(ch),
+                modifiers: 0,
+                kind,
+                repeat_count: 1,
+                generated_text: None,
+                source: ClientKeySource::WindowsConsole {
+                    record: crate::input::WindowsKeyRecord {
+                        key_down: kind == ClientKeyKind::Press,
+                        repeat_count: 1,
+                        virtual_key_code,
+                        virtual_scan_code,
+                        unicode,
+                        control_key_state,
+                    },
+                },
+            };
+            let crate::raw_input::RawInputEvent::Key(key) = event.to_raw_input_event() else {
+                panic!("captured input should remain a key");
+            };
+            let pane_event = ClientPaneInputEvent::from_terminal_key(key).expect("pane key");
+            let crate::raw_input::RawInputEvent::Key(key) =
+                pane_event.to_raw_input_event_with_windows_source(false)
+            else {
+                panic!("remote input should remain a key");
+            };
+            let bytes = runtime.encode_terminal_key(key);
+            let expected: &[u8] = if ch == '~' { b"~" } else { b"" };
+            assert_eq!(bytes, expected, "captured {ch:?} {kind:?}");
+            output.extend(bytes);
+        }
+        assert_eq!(output, b"~", "dead key must not insert its base character");
+    }
+
     #[test]
     fn client_shell_key_roundtrip_preserves_physical_generated_text_encoding() {
         let key = crate::input::TerminalKey::new(