fix: do not reuse terminal pane ids, clean up tokio tasks (#4859)

* don't reuse terminal pane_ids, perform some cleanups

* rustfmt

* add pr

4 files changed, 24 insertions, 11 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 779a92e9..c59edb1a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -83,6 +83,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 * feat: add --pane-id and --tab-id to all relevant CLI actions (https://github.com/zellij-org/zellij/pull/4846)
 * feat: add --ansi flag to all relevant CLI/plugin-api screendump commands (https://github.com/zellij-org/zellij/pull/4848)
 * fix: sixel aspect-ratio and padding issues (https://github.com/zellij-org/zellij/pull/4858)
+* fix: don't recycle pane ids for terminals (https://github.com/zellij-org/zellij/pull/4859)
 
 ## [0.43.1] - 2025-08-08
 * fix: pane rename backspace regression (https://github.com/zellij-org/zellij/pull/4346)
diff --git a/zellij-server/src/os_input_output_unix.rs b/zellij-server/src/os_input_output_unix.rs
index 052636b8..8c8ae6e0 100644
--- a/zellij-server/src/os_input_output_unix.rs
+++ b/zellij-server/src/os_input_output_unix.rs
@@ -26,7 +26,10 @@ use std::{
         process::CommandExt,
     },
     process::{Child, Command},
-    sync::{Arc, Mutex},
+    sync::{
+        atomic::{AtomicU32, Ordering},
+        Arc, Mutex,
+    },
     thread,
     time::Duration,
 };
@@ -276,6 +279,7 @@ fn handle_terminal(
 pub(crate) struct UnixPtyBackend {
     orig_termios: Arc<Mutex<Option<termios::Termios>>>,
     terminal_id_to_raw_fd: Arc<Mutex<BTreeMap<u32, Option<RawFd>>>>,
+    next_terminal_id_counter: Arc<AtomicU32>,
 }
 
 /// Try to write as many bytes from `buf` as possible to `fd` without blocking.
@@ -307,6 +311,7 @@ impl UnixPtyBackend {
         Ok(Self {
             orig_termios: Arc::new(Mutex::new(current_termios)),
             terminal_id_to_raw_fd: Arc::new(Mutex::new(BTreeMap::new())),
+            next_terminal_id_counter: Arc::new(AtomicU32::new(0)),
         })
     }
 
@@ -439,15 +444,10 @@ impl UnixPtyBackend {
     }
 
     pub fn next_terminal_id(&self) -> Option<u32> {
-        self.terminal_id_to_raw_fd
-            .lock()
-            .unwrap()
-            .keys()
-            .copied()
-            .collect::<BTreeSet<u32>>()
-            .last()
-            .map(|l| l + 1)
-            .or(Some(0))
+        Some(
+            self.next_terminal_id_counter
+                .fetch_add(1, Ordering::Relaxed),
+        )
     }
 }
 
diff --git a/zellij-server/src/pty.rs b/zellij-server/src/pty.rs
index c277dd64..bb9c800c 100644
--- a/zellij-server/src/pty.rs
+++ b/zellij-server/src/pty.rs
@@ -1761,7 +1761,9 @@ impl Pty {
         let err_context = || format!("failed to close for pane {id:?}");
         match id {
             PaneId::Terminal(id) => {
-                self.task_handles.remove(&id);
+                if let Some(handle) = self.task_handles.remove(&id) {
+                    handle.abort();
+                }
                 if let Some(child_pid) = self.id_to_child_pid.remove(&id) {
                     let err_context = || format!("failed to kill child processes for pane {id}");
                     self.bus
diff --git a/zellij-server/src/screen.rs b/zellij-server/src/screen.rs
index aa6c64da..39fde948 100644
--- a/zellij-server/src/screen.rs
+++ b/zellij-server/src/screen.rs
@@ -5924,6 +5924,16 @@ pub(crate) fn screen_thread_main(
                     },
                 }
 
+                // Clean up PTY-side resources (async reader task, child PID mapping,
+                // terminal_id_to_raw_fd entry). This is needed because the natural
+                // child exit path (quit_cb) only sends ScreenInstruction::ClosePane
+                // and never sends PtyInstruction::ClosePane. The handler in Pty is
+                // idempotent, so this is safe even if ClosePane was already sent.
+                let _ = screen
+                    .bus
+                    .senders
+                    .send_to_pty(PtyInstruction::ClosePane(id, None));
+
                 screen.log_and_report_session_state()?;
                 screen.retain_only_existing_panes_in_pane_groups();
             },