Cancel stale body fetch requests when selection changes quickly

Uses Arc<AtomicU32> to track the most recently wanted email sequence
number. The worker skips fetch requests that no longer match, avoiding
wasted network calls when scrolling through emails rapidly.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib.rs

@@ -1,5 +1,6 @@
 use std::io::{Error, Stdout};
-use std::sync::mpsc;
+use std::sync::atomic::{AtomicU32, Ordering};
+use std::sync::{mpsc, Arc};
 use std::thread;
 use std::time::{Duration, Instant};
 use crossterm::event;
@@ -49,6 +50,7 @@ fn worker_loop(
     config: Config,
     cmd_rx: mpsc::Receiver<WorkerCmd>,
     result_tx: mpsc::Sender<WorkerResult>,
+    wanted_body_seq: Arc<AtomicU32>,
 ) {
     let mut session: Option<connect::ImapSession> = None;
 
@@ -63,6 +65,10 @@ fn worker_loop(
                 let _ = result_tx.send(WorkerResult::FetchedMore(result));
             }
             WorkerCmd::FetchBody { seq } => {
+                // Skip if a newer body request has been made
+                if wanted_body_seq.load(Ordering::Relaxed) != seq {
+                    continue;
+                }
                 let result = inbox::fetch_body(&mut session, seq, &config);
                 let _ = result_tx.send(WorkerResult::Body { seq, result });
             }
@@ -83,9 +89,11 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
     let (cmd_tx, cmd_rx) = mpsc::channel();
     let (result_tx, result_rx) = mpsc::channel();
 
+    let wanted_body_seq = Arc::new(AtomicU32::new(0));
     let worker_config = config.clone();
+    let worker_wanted = Arc::clone(&wanted_body_seq);
     let worker = thread::spawn(move || {
-        worker_loop(worker_config, cmd_rx, result_tx);
+        worker_loop(worker_config, cmd_rx, result_tx, worker_wanted);
     });
 
     // Send initial refresh
@@ -129,6 +137,7 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
                             preview_body.clear();
                             preview_scroll = 0;
                             body_loading = true;
+                            wanted_body_seq.store(seq, Ordering::Relaxed);
                             let _ = cmd_tx.send(WorkerCmd::FetchBody { seq });
                         }
                     } else {
@@ -301,6 +310,7 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
                                     preview_body.clear();
                                     preview_scroll = 0;
                                     body_loading = true;
+                                    wanted_body_seq.store(seq, Ordering::Relaxed);
                                     let _ = cmd_tx.send(WorkerCmd::FetchBody { seq });
                                 }
                             }
@@ -320,6 +330,7 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
                                     preview_body.clear();
                                     preview_scroll = 0;
                                     body_loading = true;
+                                    wanted_body_seq.store(seq, Ordering::Relaxed);
                                     let _ = cmd_tx.send(WorkerCmd::FetchBody { seq });
                                 }
                             }
@@ -347,6 +358,7 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
                                         preview_body.clear();
                                         preview_scroll = 0;
                                         body_loading = true;
+                                        wanted_body_seq.store(new_seq, Ordering::Relaxed);
                                         let _ = cmd_tx.send(WorkerCmd::FetchBody { seq: new_seq });
                                     }
                                 }