Chunk terminal output below MTU

src/pty.rs

@@ -7,6 +7,12 @@ use std::sync::{Arc, Mutex};
 use std::thread;
 use tokio::sync::mpsc;
 
+// Keep live terminal output comfortably below common path MTUs after Dosh's
+// protocol header, AEAD tag, UDP/IP headers, and bincode framing. Full-screen
+// TUIs often write several KiB on the first draw; sending that as one UDP
+// datagram can fragment and vanish, leaving only a blank alternate screen.
+const PTY_OUTPUT_CHUNK_BYTES: usize = 1024;
+
 /// Backing for a PTY master held by the server.
 ///
 /// `Owned` means this process spawned the shell as a child and is responsible
@@ -264,11 +270,13 @@ fn spawn_reader_thread(
                         break;
                     }
                     Ok(n) => {
-                        let _ = tx.send(PtyOutput {
-                            session: reader_session.clone(),
-                            bytes: buf[..n].to_vec(),
-                            exited: false,
-                        });
+                        for chunk in buf[..n].chunks(PTY_OUTPUT_CHUNK_BYTES) {
+                            let _ = tx.send(PtyOutput {
+                                session: reader_session.clone(),
+                                bytes: chunk.to_vec(),
+                                exited: false,
+                            });
+                        }
                     }
                     Err(_) => {
                         let _ = tx.send(PtyOutput {
@@ -297,4 +305,9 @@ mod tests {
         assert!(!terminfo_available("definitely-not-a-real-terminal-xyz"));
         assert!(!terminfo_available(""));
     }
+
+    #[test]
+    fn pty_output_chunk_size_stays_mtu_safe() {
+        assert!(PTY_OUTPUT_CHUNK_BYTES <= 1200);
+    }
 }

tests/integration_smoke.rs

@@ -1382,6 +1382,57 @@ fn tui_control_sequences_survive_transport_verbatim() {
     }
 }
 
+#[test]
+fn large_tui_paint_is_delivered_in_mtu_safe_frames() {
+    let dir = tempfile::tempdir().unwrap();
+    let port = free_udp_port();
+    let config = write_server_config(&dir, port);
+    let mut server = start_server(&dir, &config);
+    let (socket, bootstrap, ok) = direct_attach(&config, port, "read-write");
+
+    let input = Input {
+        bytes: b"printf '\\033[?1049h'; yes DOSH_TUI_BIG_PAINT | head -n 300 | tr -d '\\n'; printf '\\033[?1049l'\n".to_vec(),
+    };
+    send_encrypted(
+        &socket,
+        port,
+        PacketKind::Input,
+        ok.client_id,
+        2,
+        0,
+        &bootstrap.session_key,
+        &protocol::to_body(&input).unwrap(),
+    );
+
+    let mut text = String::new();
+    let mut saw_split_frame = false;
+    let deadline = std::time::Instant::now() + Duration::from_secs(3);
+    while std::time::Instant::now() < deadline && text.matches("DOSH_TUI_BIG_PAINT").count() < 200
+    {
+        if let Some((_header, frame)) = recv_frame(&socket, &bootstrap.session_key) {
+            if frame.bytes.len() <= 1400 && frame.bytes.len() >= 900 {
+                saw_split_frame = true;
+            }
+            text.push_str(&String::from_utf8_lossy(&frame.bytes));
+        }
+    }
+
+    let _ = server.kill();
+    let _ = server.wait();
+
+    assert!(
+        text.contains("\x1b[?1049h") && text.matches("DOSH_TUI_BIG_PAINT").count() >= 200,
+        "large alternate-screen paint did not survive transport, enter_alt={} markers={} bytes={}",
+        text.contains("\x1b[?1049h"),
+        text.matches("DOSH_TUI_BIG_PAINT").count(),
+        text.len()
+    );
+    assert!(
+        saw_split_frame,
+        "expected large TUI paint to be split into MTU-safe frames"
+    );
+}
+
 #[test]
 fn resume_snapshot_preserves_alternate_screen_mode() {
     let dir = tempfile::tempdir().unwrap();