Fix intermittent unresponsiveness from slow/hanging IMAP connections

Three improvements:
- Add 15s connect/read/write timeouts to TcpStream so a hung IMAP
  server can no longer block the worker thread indefinitely
- Cache tui_markdown rendering: convert Text<'a> to Text<'static> on
  first render and reuse across frames, re-parsing only when the body
  actually changes
- Debounce FetchBody requests on keyboard navigation: wait 150ms of
  inactivity before sending, so rapid scrolling doesn't flood the
  worker with stale requests

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

src/connect.rs

@@ -1,7 +1,11 @@
-use std::net::TcpStream;
+use std::net::{TcpStream, ToSocketAddrs};
+use std::time::Duration;
 use native_tls::TlsStream;
 use crate::config::Config;
 
+const CONNECT_TIMEOUT: Duration = Duration::from_secs(15);
+const IO_TIMEOUT: Duration = Duration::from_secs(15);
+
 pub(crate) enum ImapSession {
     Plain(imap::Session<TcpStream>),
     Tls(imap::Session<TlsStream<TcpStream>>),
@@ -25,25 +29,30 @@ impl ImapSession {
 
 pub(crate) fn connect(config: &Config) -> Result<ImapSession, String> {
     let imap_cfg = &config.imap;
+
+    let addr = (&*imap_cfg.host, imap_cfg.port)
+        .to_socket_addrs()
+        .map_err(|e| e.to_string())?
+        .next()
+        .ok_or_else(|| "could not resolve IMAP host".to_string())?;
+
+    let tcp = TcpStream::connect_timeout(&addr, CONNECT_TIMEOUT)
+        .map_err(|e| e.to_string())?;
+    tcp.set_read_timeout(Some(IO_TIMEOUT)).map_err(|e| e.to_string())?;
+    tcp.set_write_timeout(Some(IO_TIMEOUT)).map_err(|e| e.to_string())?;
+
     if imap_cfg.use_tls {
         let tls = native_tls::TlsConnector::builder()
             .build()
             .map_err(|e| e.to_string())?;
-        let client = imap::connect(
-            (&*imap_cfg.host, imap_cfg.port),
-            &imap_cfg.host,
-            &tls,
-        )
+        let tls_stream = tls.connect(&imap_cfg.host, tcp)
             .map_err(|e| e.to_string())?;
-        let session = client
+        let session = imap::Client::new(tls_stream)
             .login(&imap_cfg.username, &imap_cfg.password)
             .map_err(|(e, _)| e.to_string())?;
         Ok(ImapSession::Tls(session))
     } else {
-        let stream =
-            TcpStream::connect((&*imap_cfg.host, imap_cfg.port)).map_err(|e| e.to_string())?;
-        let client = imap::Client::new(stream);
-        let session = client
+        let session = imap::Client::new(tcp)
             .login(&imap_cfg.username, &imap_cfg.password)
             .map_err(|(e, _)| e.to_string())?;
         Ok(ImapSession::Plain(session))

src/lib.rs

@@ -17,6 +17,7 @@ mod connect;
 mod inbox;
 
 const POLL_INTERVAL: Duration = Duration::from_secs(30);
+const NAV_DEBOUNCE: Duration = Duration::from_millis(150);
 
 #[derive(PartialEq)]
 enum Focus {
@@ -107,10 +108,13 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
     let mut list_state = ListState::default();
     let mut last_fetch = Instant::now();
     let mut message_body: String = String::new();
+    let mut message_rendered: Option<ratatui::text::Text<'static>> = None;
     let mut message_seq: Option<u32> = None;
     let mut body_loading = false;
     let mut focus = Focus::Inbox;
     let mut message_scroll: u16 = 0;
+    let mut pending_fetch_seq: Option<u32> = None;
+    let mut last_nav = Instant::now();
 
     // --- Main loop ---
     loop {
@@ -164,6 +168,7 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
                     body_loading = false;
                     if message_seq == Some(seq) {
                         message_scroll = 0;
+                        message_rendered = None;
                         match result {
                             Ok(body) => message_body = body,
                             Err(e) => message_body = format!("Error loading body: {}", e),
@@ -178,6 +183,18 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
             }
         }
 
+        // Fire pending body fetch after nav debounce elapses
+        if let Some(seq) = pending_fetch_seq {
+            if last_nav.elapsed() >= NAV_DEBOUNCE {
+                message_body.clear();
+                message_rendered = None;
+                body_loading = true;
+                wanted_body_seq.store(seq, Ordering::Relaxed);
+                let _ = cmd_tx.send(WorkerCmd::FetchBody { seq });
+                pending_fetch_seq = None;
+            }
+        }
+
         // Periodic refresh
         if !loading && last_fetch.elapsed() >= POLL_INTERVAL {
             let _ = cmd_tx.send(WorkerCmd::Refresh);
@@ -265,7 +282,24 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
             let message_text = if body_loading {
                 ratatui::text::Text::raw("Loading...")
             } else {
-                tui_markdown::from_str(&message_body)
+                message_rendered
+                    .get_or_insert_with(|| {
+                        // Convert to owned Text<'static> so it can be cached across frames
+                        let t = tui_markdown::from_str(&message_body);
+                        ratatui::text::Text {
+                            alignment: t.alignment,
+                            style: t.style,
+                            lines: t.lines.into_iter().map(|line| ratatui::text::Line {
+                                style: line.style,
+                                alignment: line.alignment,
+                                spans: line.spans.into_iter().map(|span| ratatui::text::Span {
+                                    style: span.style,
+                                    content: std::borrow::Cow::Owned(span.content.into_owned()),
+                                }).collect(),
+                            }).collect(),
+                        }
+                    })
+                    .clone()
             };
             let message_title = Line::from(Span::styled(
                 format!("{}Message", message_prefix),
@@ -317,11 +351,9 @@ pub fn main(config: &Config, terminal: &mut Terminal<CrosstermBackend<Stdout>>)
                                 let seq = emails[new_idx].seq;
                                 if message_seq != Some(seq) {
                                     message_seq = Some(seq);
-                                    message_body.clear();
                                     message_scroll = 0;
-                                    body_loading = true;
-                                    wanted_body_seq.store(seq, Ordering::Relaxed);
-                                    let _ = cmd_tx.send(WorkerCmd::FetchBody { seq });
+                                    pending_fetch_seq = Some(seq);
+                                    last_nav = Instant::now();
                                 }
                             }
                         }