fix: occasional garbage on attaching through slow connections (#5320)

* fix: occasional garbage on attaching through slow connections

* rustfmt

* add PR

5 files changed, 661 insertions, 56 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ae44d494..5c96dcc3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 * fix: live reload on theme dir changes (https://github.com/zellij-org/zellij/pull/5135)
 * feat: new title-frames UI (https://github.com/zellij-org/zellij/pull/5318)
 * chore: upgrade wasmi to 1.1.0 (https://github.com/zellij-org/zellij/pull/5319)
+* fix: fragmented STDIN causing "garbage" input on slow SSH connections (https://github.com/zellij-org/zellij/pull/5320)
 
 ## [0.44.3] - 2026-05-13
 * 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)
diff --git a/zellij-client/src/stdin_ansi_parser.rs b/zellij-client/src/stdin_ansi_parser.rs
index d6800010..d0fb4c32 100644
--- a/zellij-client/src/stdin_ansi_parser.rs
+++ b/zellij-client/src/stdin_ansi_parser.rs
@@ -204,6 +204,37 @@ pub struct ParseOutput {
 /// bounded.
 const PARTIAL_BUFFER_CAP_BYTES: usize = 100 * 1024 * 1024;
 
+const PASTE_START_MARKER: &[u8] = b"\x1b[200~";
+const PASTE_END_MARKER: &[u8] = b"\x1b[201~";
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum PendingPartial {
+    None,
+    LoneEsc,
+    ReplyInProgress,
+}
+
+#[derive(Debug, Clone, Copy)]
+enum MarkerMatch {
+    Complete,
+    Prefix,
+    No,
+}
+
+fn marker_match(buf: &[u8], marker: &[u8]) -> MarkerMatch {
+    if buf.len() >= marker.len() {
+        if &buf[..marker.len()] == marker {
+            MarkerMatch::Complete
+        } else {
+            MarkerMatch::No
+        }
+    } else if marker.starts_with(buf) {
+        MarkerMatch::Prefix
+    } else {
+        MarkerMatch::No
+    }
+}
+
 /// Outcome of a single OSC/CSI walk over a byte buffer. Distinguishing
 /// "needs more bytes" from "malformed" is what lets the residue
 /// scrubber buffer partial sequences across `feed()` calls instead of
@@ -232,6 +263,8 @@ pub struct StdinAnsiParser {
     partial_osc: Vec<u8>,
     /// Same for CSI device-control reports.
     partial_csi: Vec<u8>,
+    partial_paste: Vec<u8>,
+    in_bracketed_paste: bool,
 }
 
 impl std::fmt::Debug for StdinAnsiParser {
@@ -251,6 +284,8 @@ impl StdinAnsiParser {
             active_forward: None,
             partial_osc: Vec::new(),
             partial_csi: Vec::new(),
+            partial_paste: Vec::new(),
+            in_bracketed_paste: false,
         }
     }
 
@@ -394,20 +429,43 @@ impl StdinAnsiParser {
         // rather than leaking into residue.
         residue.extend(self.strip_replies(bytes));
         out.residue = residue;
-        out.has_partial_state = !self.partial_osc.is_empty() || !self.partial_csi.is_empty();
+        out.has_partial_state = !self.partial_osc.is_empty()
+            || !self.partial_csi.is_empty()
+            || !self.partial_paste.is_empty();
         out
     }
 
-    /// Drain any speculatively-buffered partial OSC/CSI bytes back into
-    /// keyboard residue. Called from the stdin handler's idle-timeout
-    /// path so a lone trailing ESC (or any unterminated host-reply
-    /// prefix that never received a follow-up byte) reaches the
-    /// keyboard parser instead of being stuck forever waiting for a
-    /// disambiguating byte that is never coming.
-    pub fn finalize(&mut self) -> Vec<u8> {
-        let mut out = Vec::with_capacity(self.partial_osc.len() + self.partial_csi.len());
+    pub fn pending_partial(&self) -> PendingPartial {
+        if !self.partial_paste.is_empty() {
+            PendingPartial::ReplyInProgress
+        } else if self.partial_csi.is_empty() && self.partial_osc.is_empty() {
+            PendingPartial::None
+        } else if self.partial_csi.is_empty() && self.partial_osc == [0x1b] {
+            PendingPartial::LoneEsc
+        } else {
+            PendingPartial::ReplyInProgress
+        }
+    }
+
+    pub fn finalize_lone_esc(&mut self) -> Vec<u8> {
+        if self.partial_csi.is_empty()
+            && self.partial_paste.is_empty()
+            && self.partial_osc == [0x1b]
+        {
+            std::mem::take(&mut self.partial_osc)
+        } else {
+            Vec::new()
+        }
+    }
+
+    pub fn finalize_force(&mut self) -> Vec<u8> {
+        self.in_bracketed_paste = false;
+        let mut out = Vec::with_capacity(
+            self.partial_osc.len() + self.partial_csi.len() + self.partial_paste.len(),
+        );
         out.append(&mut self.partial_osc);
         out.append(&mut self.partial_csi);
+        out.append(&mut self.partial_paste);
         out
     }
 
@@ -426,16 +484,54 @@ impl StdinAnsiParser {
         // partial_csi) is non-empty at any time — the previous walk
         // either completed all sequences or stopped at exactly one
         // unterminated tail.
-        let mut working: Vec<u8> =
-            Vec::with_capacity(self.partial_osc.len() + self.partial_csi.len() + bytes.len());
+        let mut working: Vec<u8> = Vec::with_capacity(
+            self.partial_osc.len()
+                + self.partial_csi.len()
+                + self.partial_paste.len()
+                + bytes.len(),
+        );
         working.append(&mut self.partial_osc);
         working.append(&mut self.partial_csi);
+        working.append(&mut self.partial_paste);
         working.extend_from_slice(bytes);
 
         let mut out = Vec::with_capacity(working.len());
         let mut i = 0;
         while i < working.len() {
             let rest = &working[i..];
+            if self.in_bracketed_paste {
+                if rest[0] == 0x1b {
+                    match marker_match(rest, PASTE_END_MARKER) {
+                        MarkerMatch::Complete => {
+                            out.extend_from_slice(PASTE_END_MARKER);
+                            self.in_bracketed_paste = false;
+                            i += PASTE_END_MARKER.len();
+                            continue;
+                        },
+                        MarkerMatch::Prefix => {
+                            let tail = rest.to_vec();
+                            if tail.len() > PARTIAL_BUFFER_CAP_BYTES {
+                                out.extend_from_slice(&tail);
+                            } else {
+                                self.partial_paste = tail;
+                            }
+                            return out;
+                        },
+                        MarkerMatch::No => {},
+                    }
+                }
+                out.push(working[i]);
+                i += 1;
+                continue;
+            }
+            if rest.len() >= 2 && rest[0] == 0x1b && rest[1] == b'[' {
+                if let MarkerMatch::Complete = marker_match(rest, PASTE_START_MARKER) {
+                    out.extend_from_slice(PASTE_START_MARKER);
+                    self.in_bracketed_paste = true;
+                    i += PASTE_START_MARKER.len();
+                    continue;
+                }
+            }
             // OSC: ESC ] ... (BEL | ESC \)
             if rest.len() >= 2 && rest[0] == 0x1b && rest[1] == b']' {
                 match osc_status(rest) {
diff --git a/zellij-client/src/stdin_ansi_parser_tests.rs b/zellij-client/src/stdin_ansi_parser_tests.rs
index b34aa343..19fd5698 100644
--- a/zellij-client/src/stdin_ansi_parser_tests.rs
+++ b/zellij-client/src/stdin_ansi_parser_tests.rs
@@ -1,6 +1,6 @@
 //! Unit tests for the continuous host-reply parser.
 
-use super::{schedule_forward_timeout, HostReply, StdinAnsiParser};
+use super::{schedule_forward_timeout, HostReply, PendingPartial, StdinAnsiParser};
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
 
@@ -308,14 +308,16 @@ fn lone_trailing_esc_is_buffered_then_finalized_as_residue() {
         out.has_partial_state,
         "lone ESC must mark has_partial_state so the caller schedules a finalize tick"
     );
-    let drained = p.finalize();
+    assert_eq!(p.pending_partial(), PendingPartial::LoneEsc);
+    let drained = p.finalize_lone_esc();
     assert_eq!(
         drained,
         vec![0x1b],
         "finalize must release the parked ESC as keyboard residue"
     );
     // Subsequent finalize is a no-op once the parker is empty.
-    assert!(p.finalize().is_empty());
+    assert!(p.finalize_lone_esc().is_empty());
+    assert_eq!(p.pending_partial(), PendingPartial::None);
 }
 
 #[test]
@@ -333,7 +335,10 @@ fn fragmented_osc_does_not_finalize_partial() {
     let r2 = p.feed(&full[1..]);
     assert!(r2.residue.is_empty(), "tail must complete the OSC");
     assert_eq!(r1.replies.len() + r2.replies.len(), 1);
-    assert!(p.finalize().is_empty(), "no partial left after completion");
+    assert!(
+        p.finalize_force().is_empty(),
+        "no partial left after completion"
+    );
 }
 
 #[test]
@@ -875,3 +880,348 @@ fn kitty_kbd_event_does_not_wedge_subsequent_forward_reply() {
         );
     }
 }
+
+use crate::keyboard_parser::{KittyKeyboardParser, KittyParseOutcome};
+use zellij_utils::data::BareKey;
+
+fn assert_held_across_idle(parser: &mut StdinAnsiParser) {
+    assert_eq!(
+        parser.pending_partial(),
+        PendingPartial::ReplyInProgress,
+        "a payload-bearing partial must report ReplyInProgress while held"
+    );
+    assert!(
+        parser.finalize_lone_esc().is_empty(),
+        "the short idle finalize must NOT drain a reply-in-progress partial"
+    );
+    assert_eq!(
+        parser.pending_partial(),
+        PendingPartial::ReplyInProgress,
+        "the partial must still be held after a lone-esc finalize"
+    );
+}
+
+#[test]
+fn fragmented_osc4_reply_is_retained_not_leaked() {
+    let mut p = StdinAnsiParser::new();
+    let r1 = p.feed(b"\x1b]4;1;rgb:1111");
+    assert!(
+        r1.residue.is_empty(),
+        "incomplete OSC 4 must not leak: {:?}",
+        r1.residue
+    );
+    assert!(r1.replies.is_empty());
+    assert_held_across_idle(&mut p);
+
+    let r2 = p.feed(b"/2222/3333\x1b\\");
+    assert!(
+        r2.residue.is_empty(),
+        "completion must not leak: {:?}",
+        r2.residue
+    );
+    assert_eq!(r2.replies.len(), 1, "the rejoined OSC 4 classifies once");
+    match &r2.replies[0] {
+        HostReply::ColorRegisters(regs) => {
+            assert_eq!(regs[0].0, 1);
+            assert_eq!(regs[0].1, "rgb:1111/2222/3333");
+        },
+        other => panic!("expected ColorRegisters, got {:?}", other),
+    }
+    assert_eq!(p.pending_partial(), PendingPartial::None);
+}
+
+#[test]
+fn fragmented_osc11_reply_is_retained_across_idle() {
+    let mut p = StdinAnsiParser::new();
+    let r1 = p.feed(b"\x1b]11;rgb:0000/00");
+    assert!(r1.residue.is_empty());
+    assert_held_across_idle(&mut p);
+
+    let r2 = p.feed(b"00/0000\x1b\\");
+    assert!(r2.residue.is_empty());
+    assert_eq!(r2.replies.len(), 1);
+    matches!(r2.replies[0], HostReply::BackgroundColor(_));
+}
+
+#[test]
+fn esc_companion_chord_is_not_held_as_reply() {
+    let mut whole = StdinAnsiParser::new();
+    let out = whole.feed(b"\x1ba");
+    assert_eq!(out.residue, b"\x1ba");
+    assert!(out.replies.is_empty());
+
+    let mut split = StdinAnsiParser::new();
+    let r1 = split.feed(b"\x1b");
+    assert!(
+        r1.residue.is_empty(),
+        "lone ESC must park until disambiguated"
+    );
+    assert_eq!(split.pending_partial(), PendingPartial::LoneEsc);
+    let r2 = split.feed(b"a");
+    let mut combined = r1.residue.clone();
+    combined.extend_from_slice(&r2.residue);
+    assert_eq!(combined, b"\x1ba");
+    assert!(r2.replies.is_empty());
+}
+
+#[test]
+fn fragmented_ctrl_arrow_is_retained_then_passed_through() {
+    let mut p = StdinAnsiParser::new();
+    let r1 = p.feed(b"\x1b[1;5");
+    assert!(r1.residue.is_empty(), "incomplete CSI must not leak");
+    assert_held_across_idle(&mut p);
+
+    let r2 = p.feed(b"C");
+    assert_eq!(r2.residue, b"\x1b[1;5C", "the arrow reassembles intact");
+    assert!(r2.replies.is_empty(), "a cursor key is never a host reply");
+}
+
+#[test]
+fn fragmented_sgr_mouse_is_retained_then_passed_through() {
+    let mut p = StdinAnsiParser::new();
+    let r1 = p.feed(b"\x1b[<0;12;3");
+    assert!(r1.residue.is_empty());
+    assert_held_across_idle(&mut p);
+    let r2 = p.feed(b"4M");
+    assert_eq!(r2.residue, b"\x1b[<0;12;34M");
+    assert!(r2.replies.is_empty());
+}
+
+#[test]
+fn focus_event_is_residue_not_reply() {
+    let mut whole = StdinAnsiParser::new();
+    let out = whole.feed(b"\x1b[I");
+    assert_eq!(out.residue, b"\x1b[I");
+    assert!(out.replies.is_empty());
+
+    let mut split = StdinAnsiParser::new();
+    let r1 = split.feed(b"\x1b[");
+    assert!(r1.residue.is_empty());
+    let r2 = split.feed(b"I");
+    let mut combined = r1.residue.clone();
+    combined.extend_from_slice(&r2.residue);
+    assert_eq!(combined, b"\x1b[I");
+    assert!(r2.replies.is_empty());
+}
+
+#[test]
+fn kitty_keyboard_variants_whole_and_fragmented() {
+    let variants: Vec<&[u8]> = vec![
+        b"\x1b[97u",
+        b"\x1b[97;5u",
+        b"\x1b[97;1:3u",
+        b"\x1b[97:65;5u",
+        b"\x1b[97;5;65u",
+        b"\x1b[1;5A",
+        b"\x1b[1;2H",
+        b"\x1b[1;2P",
+        b"\x1b[1;2S",
+        b"\x1b[2;5~",
+        b"\x1b[3;5~",
+        b"\x1b[15;2~",
+        b"\x1b[27u",
+        b"\x1b[57362u",
+        b"\x1b[Z",
+    ];
+    for seq in variants {
+        let pretty = String::from_utf8_lossy(seq).replace('\x1b', "ESC");
+
+        let mut whole = StdinAnsiParser::new();
+        let out = whole.feed(seq);
+        assert_eq!(
+            out.residue, seq,
+            "{}: whole residue must equal the sequence",
+            pretty
+        );
+        assert!(out.replies.is_empty(), "{}: a key is never a reply", pretty);
+
+        for split in 2..seq.len() {
+            let mut p = StdinAnsiParser::new();
+            let r1 = p.feed(&seq[..split]);
+            assert!(
+                r1.residue.is_empty(),
+                "{} split at {}: prefix must be held, not leaked: {:?}",
+                pretty,
+                split,
+                r1.residue
+            );
+            assert_eq!(
+                p.pending_partial(),
+                PendingPartial::ReplyInProgress,
+                "{} split at {}: prefix must be held",
+                pretty,
+                split
+            );
+            assert!(
+                p.finalize_lone_esc().is_empty(),
+                "{} split at {}: idle must not drain the held key",
+                pretty,
+                split
+            );
+            let r2 = p.feed(&seq[split..]);
+            let mut combined = r1.residue.clone();
+            combined.extend_from_slice(&r2.residue);
+            assert_eq!(
+                combined, seq,
+                "{} split at {}: reassembled residue must equal the sequence",
+                pretty, split
+            );
+            assert!(
+                r1.replies.is_empty() && r2.replies.is_empty(),
+                "{} split at {}: never classified as a reply",
+                pretty,
+                split
+            );
+        }
+    }
+}
+
+#[test]
+fn kitty_ctrl_a_trace_yields_one_event_not_a_literal_u() {
+    let mut p = StdinAnsiParser::new();
+    let r1 = p.feed(b"\x1b[97;5");
+    assert!(r1.residue.is_empty());
+    assert_held_across_idle(&mut p);
+    let r2 = p.feed(b"u");
+    assert_eq!(r2.residue, b"\x1b[97;5u");
+
+    let mut kitty = KittyKeyboardParser::new();
+    match kitty.feed(&r2.residue) {
+        KittyParseOutcome::Complete(key) => {
+            assert!(
+                key.is_key_with_ctrl_modifier(BareKey::Char('a')),
+                "expected Ctrl-a, got {:?}",
+                key
+            );
+        },
+        other => panic!("expected a complete Ctrl-a, got {:?}", other),
+    }
+}
+
+#[test]
+fn bracketed_paste_preserves_embedded_escape_content() {
+    let mut p = StdinAnsiParser::new();
+    let mut chunk = Vec::new();
+    chunk.extend_from_slice(b"\x1b[200~");
+    chunk.extend_from_slice(b"echo \x1b]0;hi\x1b\\ done");
+    chunk.extend_from_slice(b"\x1b[201~");
+    let out = p.feed(&chunk);
+    assert!(
+        out.replies.is_empty(),
+        "paste content must not classify as a reply: {:?}",
+        out.replies
+    );
+    assert!(
+        out.desktop_notifications.is_empty(),
+        "an OSC inside a paste is literal, not a notification"
+    );
+    assert_eq!(
+        out.residue, chunk,
+        "the entire paste body, including the embedded OSC, passes through verbatim"
+    );
+    assert!(
+        out.residue.windows(6).any(|w| w == b"\x1b]0;hi"),
+        "the embedded OSC bytes must survive in residue"
+    );
+}
+
+#[test]
+fn paste_embedded_osc_is_not_eaten_when_fragmented() {
+    let full: Vec<u8> = b"\x1b[200~A\x1b]11;rgb:0/0/0\x1b\\B\x1b[201~".to_vec();
+    for split in 1..full.len() {
+        let mut p = StdinAnsiParser::new();
+        let r1 = p.feed(&full[..split]);
+        let r2 = p.feed(&full[split..]);
+        let mut combined = r1.residue.clone();
+        combined.extend_from_slice(&r2.residue);
+        assert_eq!(
+            combined, full,
+            "split at {}: paste body (with embedded OSC) must pass through whole",
+            split
+        );
+        assert!(
+            r1.replies.is_empty() && r2.replies.is_empty(),
+            "split at {}: embedded OSC must not classify as a reply",
+            split
+        );
+    }
+}
+
+#[test]
+fn paste_markers_fragmented_still_toggle_paste_mode() {
+    for (head, tail) in [(b"\x1b[20".as_ref(), b"0~".as_ref())] {
+        let mut p = StdinAnsiParser::new();
+        let r1 = p.feed(head);
+        assert!(r1.residue.is_empty(), "fragmented start marker is held");
+        let r2 = p.feed(tail);
+        let mut start = r1.residue.clone();
+        start.extend_from_slice(&r2.residue);
+        assert_eq!(
+            start, b"\x1b[200~",
+            "start marker passes through to residue"
+        );
+
+        let body = p.feed(b"\x1b]0;x\x1b\\");
+        assert_eq!(body.residue, b"\x1b]0;x\x1b\\");
+        assert!(body.replies.is_empty());
+    }
+    for (head, tail) in [(b"\x1b[20".as_ref(), b"1~".as_ref())] {
+        let mut p = StdinAnsiParser::new();
+        let _ = p.feed(b"\x1b[200~body");
+        let r1 = p.feed(head);
+        let r2 = p.feed(tail);
+        let mut end = r1.residue.clone();
+        end.extend_from_slice(&r2.residue);
+        assert_eq!(end, b"\x1b[201~", "end marker passes through to residue");
+
+        let after = p.feed(b"\x1b]11;rgb:1/2/3\x1b\\");
+        assert!(after.residue.is_empty());
+        assert_eq!(after.replies.len(), 1);
+    }
+}
+
+#[test]
+fn last_resort_force_drain_recovers_a_never_terminated_partial() {
+    let mut p = StdinAnsiParser::new();
+    let r1 = p.feed(b"\x1b]4;1;rgb:incomplete");
+    assert!(r1.residue.is_empty());
+    assert_eq!(p.pending_partial(), PendingPartial::ReplyInProgress);
+
+    let r2 = p.feed(b"and-more");
+    assert!(r2.residue.is_empty());
+    assert_eq!(p.pending_partial(), PendingPartial::ReplyInProgress);
+
+    let drained = p.finalize_force();
+    assert!(
+        drained.windows(14).any(|w| w == b"rgb:incomplete"),
+        "force-drain must recover the buffered partial: {:?}",
+        drained
+    );
+    assert!(
+        drained.windows(8).any(|w| w == b"and-more"),
+        "force-drain must recover bytes appended before the guard: {:?}",
+        drained
+    );
+    assert_eq!(
+        p.pending_partial(),
+        PendingPartial::None,
+        "the buffer is empty after a force-drain"
+    );
+}
+
+#[test]
+fn unsolicited_theme_notification_classifies_without_outstanding_query() {
+    let mut p = StdinAnsiParser::new();
+    let out = p.feed(b"\x1b[?997;1n");
+    assert!(
+        out.residue.is_empty(),
+        "the notification must not leak as residue"
+    );
+    assert_eq!(out.replies.len(), 1);
+    match &out.replies[0] {
+        HostReply::HostTerminalThemeChanged(mode) => {
+            assert_eq!(*mode, zellij_utils::data::HostTerminalThemeMode::Dark);
+        },
+        other => panic!("expected HostTerminalThemeChanged, got {:?}", other),
+    }
+}
diff --git a/zellij-client/src/stdin_handler.rs b/zellij-client/src/stdin_handler.rs
index c33e0dbf..26418188 100644
--- a/zellij-client/src/stdin_handler.rs
+++ b/zellij-client/src/stdin_handler.rs
@@ -1,11 +1,14 @@
 use crate::keyboard_parser::{KittyKeyboardParser, KittyParseOutcome};
 use crate::os_input_output::ClientOsApi;
-use crate::stdin_ansi_parser::StdinAnsiParser;
+use crate::stdin_ansi_parser::{PendingPartial, StdinAnsiParser};
 #[cfg(windows)]
 use crate::stdin_handler_windows::enable_vt_input;
 use crate::InputInstruction;
 use std::sync::{mpsc, Arc, Mutex};
-use std::time::Duration;
+use std::time::{Duration, Instant};
+
+const LONE_ESC_FLUSH_INTERVAL: Duration = Duration::from_millis(50);
+const PARTIAL_REPLY_FLUSH_GUARD: Duration = Duration::from_millis(1000);
 use zellij_utils::{
     channels::SenderWithContext,
     vendored::termwiz::input::{InputEvent, InputParser},
@@ -99,9 +102,10 @@ pub(crate) fn stdin_loop(
             }
         });
     let mut needs_finalization = false;
+    let mut reply_in_progress_since: Option<Instant> = None;
     loop {
         match if needs_finalization {
-            stdin_rx.recv_timeout(Duration::from_millis(50))
+            stdin_rx.recv_timeout(LONE_ESC_FLUSH_INTERVAL)
         } else {
             stdin_rx
                 .recv()
@@ -134,20 +138,14 @@ pub(crate) fn stdin_loop(
                             let _ = send_input_instructions
                                 .send(InputInstruction::DesktopNotificationResponse(payload));
                         }
-                        let has_partial = parse_output.has_partial_state;
                         let residue = parse_output.residue;
                         if residue.is_empty() {
-                            // If all bytes were consumed by the host-reply
-                            // parser, nothing to feed to the keyboard
-                            // parser. But if the host-reply parser is
-                            // sitting on a buffered partial sequence
-                            // (e.g. a lone trailing ESC waiting to be
-                            // disambiguated), schedule a finalize tick
-                            // so the idle drain releases it as keyboard
-                            // residue when no follow-up arrives.
-                            if has_partial {
-                                needs_finalization = true;
-                            }
+                            schedule_finalization(
+                                &stdin_ansi_parser,
+                                false,
+                                &mut needs_finalization,
+                                &mut reply_in_progress_since,
+                            );
                             continue;
                         }
                         current_buffer.append(&mut residue.clone());
@@ -168,6 +166,12 @@ pub(crate) fn stdin_loop(
                                             true,
                                         ))
                                         .unwrap();
+                                    schedule_finalization(
+                                        &stdin_ansi_parser,
+                                        false,
+                                        &mut needs_finalization,
+                                        &mut reply_in_progress_since,
+                                    );
                                     continue;
                                 },
                                 KittyParseOutcome::Incomplete | KittyParseOutcome::NoMatch => {},
@@ -201,7 +205,12 @@ pub(crate) fn stdin_loop(
                                 .unwrap();
                         }
 
-                        needs_finalization = true;
+                        schedule_finalization(
+                            &stdin_ansi_parser,
+                            true,
+                            &mut needs_finalization,
+                            &mut reply_in_progress_since,
+                        );
                     },
                     Err(e) => {
                         if e == "Session ended" {
@@ -215,13 +224,38 @@ pub(crate) fn stdin_loop(
                 }
             },
             Err(mpsc::RecvTimeoutError::Timeout) => {
-                finalize_events(
-                    &mut input_parser,
-                    &mut current_buffer,
-                    send_input_instructions.clone(),
-                    &stdin_ansi_parser,
-                );
-                needs_finalization = false;
+                let pending = stdin_ansi_parser.lock().unwrap().pending_partial();
+                match pending {
+                    PendingPartial::ReplyInProgress => {
+                        let elapsed = reply_in_progress_since
+                            .map(|since| since.elapsed())
+                            .unwrap_or_default();
+                        if elapsed >= PARTIAL_REPLY_FLUSH_GUARD {
+                            let drained = stdin_ansi_parser.lock().unwrap().finalize_force();
+                            drain_partial_to_keyboard(
+                                &mut input_parser,
+                                &mut current_buffer,
+                                send_input_instructions.clone(),
+                                drained,
+                            );
+                            needs_finalization = false;
+                            reply_in_progress_since = None;
+                        } else {
+                            needs_finalization = true;
+                        }
+                    },
+                    _ => {
+                        let drained = stdin_ansi_parser.lock().unwrap().finalize_lone_esc();
+                        drain_partial_to_keyboard(
+                            &mut input_parser,
+                            &mut current_buffer,
+                            send_input_instructions.clone(),
+                            drained,
+                        );
+                        needs_finalization = false;
+                        reply_in_progress_since = None;
+                    },
+                }
             },
             Err(mpsc::RecvTimeoutError::Disconnected) => {
                 log::debug!("STDIN pump disconnected");
@@ -232,18 +266,31 @@ pub(crate) fn stdin_loop(
     }
 }
 
-fn finalize_events(
+fn schedule_finalization(
+    stdin_ansi_parser: &Arc<Mutex<StdinAnsiParser>>,
+    fed_termwiz: bool,
+    needs_finalization: &mut bool,
+    reply_in_progress_since: &mut Option<Instant>,
+) {
+    let pending = stdin_ansi_parser.lock().unwrap().pending_partial();
+    if fed_termwiz || pending != PendingPartial::None {
+        *needs_finalization = true;
+    }
+    if pending == PendingPartial::ReplyInProgress {
+        if reply_in_progress_since.is_none() {
+            *reply_in_progress_since = Some(Instant::now());
+        }
+    } else {
+        *reply_in_progress_since = None;
+    }
+}
+
+fn drain_partial_to_keyboard(
     input_parser: &mut InputParser,
     current_buffer: &mut Vec<u8>,
     send_input_instructions: SenderWithContext<InputInstruction>,
-    stdin_ansi_parser: &Arc<Mutex<StdinAnsiParser>>,
+    drained: Vec<u8>,
 ) {
-    // Drain any speculatively-buffered partial host-reply bytes (a
-    // lone trailing ESC, or an unterminated OSC/CSI prefix whose
-    // follow-up never arrived). They become keyboard residue — same
-    // path real keypress bytes take. Without this drain, a real Esc
-    // press whose byte was parked under partial_osc would be lost.
-    let drained = stdin_ansi_parser.lock().unwrap().finalize();
     if !drained.is_empty() {
         current_buffer.extend_from_slice(&drained);
     }
@@ -256,8 +303,6 @@ fn finalize_events(
         },
         false,
     );
-    // Residue contains no OSC or whitelisted CSI reports — every
-    // termwiz event drained on idle is a key/mouse/paste/etc.
     for input_event in events {
         send_input_instructions
             .send(InputInstruction::KeyEvent(
@@ -277,13 +322,24 @@ fn build_startup_query_string() -> String {
     // <ESC>]11;?<ESC>\ => get background color
     // <ESC>]10;?<ESC>\ => get foreground color
     // <ESC>[?2026$p => get synchronised output mode
-    let mut query_string = String::from(
-        "\u{1b}[14t\u{1b}[16t\u{1b}]11;?\u{1b}\u{5c}\u{1b}]10;?\u{1b}\u{5c}\u{1b}[?2026$p",
-    );
-    // query colors
-    // eg. <ESC>]4;5;?<ESC>\ => query color register number 5
-    for i in 0..256 {
-        query_string.push_str(&format!("\u{1b}]4;{};?\u{1b}\u{5c}", i));
+    String::from("\u{1b}[14t\u{1b}[16t\u{1b}]11;?\u{1b}\u{5c}\u{1b}]10;?\u{1b}\u{5c}\u{1b}[?2026$p")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::build_startup_query_string;
+
+    #[test]
+    fn startup_query_has_no_palette_register_loop() {
+        let query = build_startup_query_string();
+        assert_eq!(
+            query,
+            "\u{1b}[14t\u{1b}[16t\u{1b}]11;?\u{1b}\u{5c}\u{1b}]10;?\u{1b}\u{5c}\u{1b}[?2026$p"
+        );
+        assert!(
+            !query.contains("\u{1b}]4;"),
+            "startup query must not contain OSC 4 palette-register probes: {:?}",
+            query
+        );
     }
-    query_string
 }
diff --git a/zellij-integration-tests/tests/startup_host_query.rs b/zellij-integration-tests/tests/startup_host_query.rs
new file mode 100644
index 00000000..514df24f
--- /dev/null
+++ b/zellij-integration-tests/tests/startup_host_query.rs
@@ -0,0 +1,102 @@
+#![cfg(unix)]
+
+use std::thread::sleep;
+use std::time::Duration;
+
+use zellij_integration_tests::{
+    claim_first_terminal_and_wait_for_prompt, keys, start_zellij, TestSession,
+};
+
+const IDLE_BEYOND_SHORT_FLUSH: Duration = Duration::from_millis(150);
+
+fn lock_interface(zellij: &TestSession) {
+    zellij.send_stdin(&keys::ctrl('g'));
+    zellij.wait_until("interface locked", |grid_snapshot| {
+        grid_snapshot.contains("LOCK") && !grid_snapshot.contains("PANE")
+    });
+}
+
+fn unlock_interface(zellij: &TestSession) {
+    zellij.send_stdin(&keys::ctrl('g'));
+    zellij.wait_until("interface unlocked", |grid_snapshot| {
+        grid_snapshot.contains("PANE")
+    });
+}
+
+#[test]
+fn fragmented_attach_reply_burst_does_not_leak_into_focused_pane() {
+    let mut zellij = start_zellij();
+    let terminal = claim_first_terminal_and_wait_for_prompt(&zellij);
+    lock_interface(&zellij);
+
+    zellij.send_stdin(b"\x1b]4;1;rgb:abcd");
+    sleep(IDLE_BEYOND_SHORT_FLUSH);
+    zellij.send_stdin(b"/ef01/2345\x1b\\");
+
+    zellij.send_stdin(b"Z");
+    let stdin = terminal.wait_for_stdin("sentinel keystroke reached the pane", |stdin_bytes| {
+        stdin_bytes.contains(&b'Z')
+    });
+
+    assert!(
+        !stdin.windows(4).any(|window| window == b"rgb:"),
+        "host reply bytes leaked into the pane: {:?}",
+        stdin
+    );
+    assert!(
+        !stdin.contains(&b'/'),
+        "host reply payload leaked into the pane: {:?}",
+        stdin
+    );
+    assert!(
+        !stdin.contains(&b';'),
+        "host reply payload leaked into the pane: {:?}",
+        stdin
+    );
+
+    unlock_interface(&zellij);
+    zellij.quit();
+}
+
+#[test]
+fn fragmented_function_key_arrives_intact() {
+    let mut zellij = start_zellij();
+    let terminal = claim_first_terminal_and_wait_for_prompt(&zellij);
+    lock_interface(&zellij);
+
+    zellij.send_stdin(b"\x1b[1;5");
+    sleep(IDLE_BEYOND_SHORT_FLUSH);
+    zellij.send_stdin(b"C");
+
+    terminal.wait_for_stdin("ctrl-right reached the pane intact", |stdin_bytes| {
+        stdin_bytes.windows(6).any(|window| window == b"\x1b[1;5C")
+    });
+
+    unlock_interface(&zellij);
+    zellij.quit();
+}
+
+#[test]
+fn normal_typing_is_not_delayed_or_duplicated() {
+    let mut zellij = start_zellij();
+    let terminal = claim_first_terminal_and_wait_for_prompt(&zellij);
+    lock_interface(&zellij);
+
+    zellij.send_stdin(b"hello");
+    let stdin = terminal.wait_for_stdin("typed keys reached the pane", |stdin_bytes| {
+        stdin_bytes.windows(5).any(|window| window == b"hello")
+    });
+
+    let occurrences = stdin
+        .windows(5)
+        .filter(|window| *window == b"hello")
+        .count();
+    assert_eq!(
+        occurrences, 1,
+        "typed keys must reach the pane exactly once: {:?}",
+        stdin
+    );
+
+    unlock_interface(&zellij);
+    zellij.quit();
+}