fix: occasional stall when querying host terminal (introduced in 0.44.1) (#5149)

* fix(client): unwedge termwiz on unrecognized CSI sequences

* rustfmt

* add pr

3 files changed, 127 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 058c29c7..7455a21d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ## [Unreleased]
 * feat: allow tabs to have different sizes if clients aren't focused on the same one (https://github.com/zellij-org/zellij/pull/5133)
 * fix(windows): bump windows-sys to 0.59 to align manifest with code, fixing source builds via `cargo install`/`cargo binstall` (https://github.com/zellij-org/zellij/pull/5139)
+* fix: occasional stall before prompt/program-run (https://github.com/zellij-org/zellij/pull/5149)
 
 ## [0.44.2] - 2026-05-05
 * fix: idle CPU + disk i/o, CommandChanged plugin Event, GetSessionList plugin command (https://github.com/zellij-org/zellij/pull/5063)
diff --git a/zellij-client/src/stdin_ansi_parser_tests.rs b/zellij-client/src/stdin_ansi_parser_tests.rs
index 0c3b442e..b34aa343 100644
--- a/zellij-client/src/stdin_ansi_parser_tests.rs
+++ b/zellij-client/src/stdin_ansi_parser_tests.rs
@@ -817,3 +817,61 @@ fn host_theme_dsr_997_unknown_param_dropped() {
         "unknown ?997;N value must not classify"
     );
 }
+
+// =====================================================================
+// Regression test: a Kitty keyboard-protocol event arriving on the
+// client's stdin must not wedge the parser. Before the fix, such an
+// event (`\x1b[<keycode>;<mods>u`) sat at the head of termwiz's
+// internal buffer because parse_csi_report rejects the unwhitelisted
+// final byte 'u' AND the keymap returned `Found::NeedData` (it's a
+// possible prefix of registered key sequences). Every host reply that
+// arrived behind it was silently swallowed, stalling host-color
+// forwards until session timeout. This was confirmed via WEDGE-CANDIDATE
+// log entries from three independent users on Kitty, Ghostty, and
+// Konsole — all triggered by the same `\x1b[<digits>;<digits>u` shape.
+//
+// Captured triggers from real logs:
+//   `\x1b[57414;129u`  (Kitty user, repro-logs/third-commit/zellij(4).log)
+//   `\x1b[27;129u`     (Ghostty user, ...zellij(5).log)
+//   `\x1b[102;133u`    (Konsole user, ...zellij_0.45_new-2.log)
+// =====================================================================
+
+#[test]
+fn kitty_kbd_event_does_not_wedge_subsequent_forward_reply() {
+    let kbd_events: Vec<&[u8]> = vec![
+        b"\x1b[57414;129u", // captured from Kitty user
+        b"\x1b[27;129u",    // captured from Ghostty user
+        b"\x1b[102;133u",   // captured from Konsole user
+        b"\x1b[A",          // legacy CSI key (not Kitty kbd) — sanity
+    ];
+
+    for kbd in &kbd_events {
+        let pretty = String::from_utf8_lossy(kbd).replace('\x1b', "ESC");
+        let mut parser = StdinAnsiParser::new();
+
+        // Pre-wedge: keypress arrives before the forward goes out.
+        let _ = parser.feed(kbd);
+
+        // Forward dispatched — slot opens for token=42.
+        parser.open_forward(42);
+
+        // Host's reply arrives in two chunks (matches captured wire:
+        // OSC 11 reply 25B, then DA1 barrier 10B).
+        let _ = parser.feed(b"\x1b]11;rgb:1e1e/1e1e/2e2e\x1b\\");
+        let out = parser.feed(b"\x1b[?62;52;c");
+
+        let (token, bytes) = out.completed_forward.unwrap_or_else(|| {
+            panic!(
+                "BARRIER never closed slot for kbd-event prelude {:?} — \
+                 termwiz wedge regression",
+                pretty
+            )
+        });
+        assert_eq!(token, 42, "wrong token closed for prelude {:?}", pretty);
+        assert!(
+            !bytes.is_empty(),
+            "OSC payload was lost behind the kbd event for prelude {:?}",
+            pretty
+        );
+    }
+}
diff --git a/zellij-utils/src/vendored/termwiz/input.rs b/zellij-utils/src/vendored/termwiz/input.rs
index c25f712b..ec84a77f 100644
--- a/zellij-utils/src/vendored/termwiz/input.rs
+++ b/zellij-utils/src/vendored/termwiz/input.rs
@@ -838,6 +838,38 @@ fn parse_sgr_mouse(buf: &[u8]) -> Option<(InputEvent, usize)> {
 /// `ESC [` for two distinct cases — not currently disambiguated here:
 /// - truly malformed / unsupported sequence, or
 /// - incomplete input; caller handles incompleteness via `maybe_more`.
+/// Return `Some(len)` if `buf` starts with a structurally complete CSI
+/// sequence (`\x1b[ <params>* <intermediates>* <final>` per ECMA-48 §5.4),
+/// regardless of whether the final byte is one we have a use for. Used by
+/// `process_bytes` to advance past CSI sequences the keymap doesn't
+/// recognise — most importantly Kitty keyboard-protocol events
+/// `\x1b[<keycode>;<mods>u`. Without this, the keymap returns
+/// `Found::NeedData` (it sees the bytes as a possible prefix of a longer
+/// registered key) and the parser wedges holding bytes that will never
+/// extend into anything.
+///
+/// Returns `None` if the buffer doesn't start with `\x1b[`, contains a
+/// non-CSI byte, or hasn't yet received its final byte.
+fn complete_csi_len(buf: &[u8]) -> Option<usize> {
+    if buf.get(0) != Some(&0x1b) || buf.get(1) != Some(&b'[') {
+        return None;
+    }
+    let mut i = 2;
+    let max_scan = buf.len().min(256);
+    while i < max_scan {
+        let b = buf[i];
+        match b {
+            // Parameters (digits, ;, :, ?, <, =, >) and intermediates (space..'/').
+            0x30..=0x3F | 0x20..=0x2F => i += 1,
+            // Any byte in the final-byte range terminates a CSI sequence.
+            0x40..=0x7E => return Some(i + 1),
+            // Anything else means this isn't a well-formed CSI.
+            _ => return None,
+        }
+    }
+    None
+}
+
 fn parse_csi_report(buf: &[u8]) -> Option<(InputEvent, usize)> {
     if buf.get(0) != Some(&0x1b) || buf.get(1) != Some(&b'[') {
         return None;
@@ -1701,6 +1733,42 @@ impl InputParser {
                             self.buf.advance(len);
                         },
                         (Found::Ambiguous(_, _), true) | (Found::NeedData, true) => {
+                            // The keymap is signalling "this buffer
+                            // could still grow into a registered key,
+                            // give me more bytes." That verdict is
+                            // wrong when the buffer already holds a
+                            // structurally complete CSI sequence whose
+                            // final byte isn't in the keymap — most
+                            // importantly Kitty keyboard-protocol
+                            // events `\x1b[<keycode>;<mods>u`, which
+                            // never grow into anything the keymap
+                            // knows. Returning here would wedge
+                            // `self.buf` indefinitely, swallowing every
+                            // host reply that arrives behind it (the
+                            // OSC + DA1 bytes for a forwarded
+                            // `OSC 11;?` query among them) and stalling
+                            // host-color forwards until session exit.
+                            //
+                            // Both `Ambiguous(_, true)` and
+                            // `NeedData(true)` reach this point in
+                            // practice: for `\x1b[<digits>;<digits>u`
+                            // the trie reports `Ambiguous(1, Escape)`
+                            // (it has ESC alone as a match and ESC[…]
+                            // as longer prefixes), so the fix must
+                            // cover both verdicts.
+                            //
+                            // Skip past the unrecognised CSI without
+                            // emitting an event; callers that need
+                            // keyboard dispatch (kitty_parser, the
+                            // separate `input_parser` instance fed the
+                            // residue from `StdinAnsiParser`) see the
+                            // same bytes via `strip_replies`, which
+                            // already treats unwhitelisted-final CSIs
+                            // as `Malformed` and pushes them through.
+                            if let Some(len) = complete_csi_len(self.buf.as_slice()) {
+                                self.buf.advance(len);
+                                continue;
+                            }
                             return;
                         },
                         (Found::None, _) | (Found::NeedData, false) => {