fix(ui): coalesce outbound transfer list refreshes

Every outbound transfer start and finish can arrive on a hot path while a peer
is serving many file chunks. The Tauri event handler used to rebuild and emit
the full games list for each edge, cloning all games and probing per-game server
script files repeatedly during an active serve.

Batch outbound-transfer count changes behind a short scheduled refresh. The
peer still records exact counts in shared state, and the delayed refresh reads
that state once per burst. A generation counter keeps changes that arrive while
an emit is already scheduled from being lost; they trigger one follow-up emit
with the latest counts.

Test Plan:
- just test
- just clippy
- git diff --check

Refs: Claude review finding #2

crates/lanspread-tauri-deno-ts/src-tauri/src/lib.rs

@@ -3,7 +3,7 @@ use std::{
     net::SocketAddr,
     path::{Component, Path, PathBuf},
     sync::{Arc, OnceLock},
-    time::{SystemTime, UNIX_EPOCH},
+    time::{Duration, SystemTime, UNIX_EPOCH},
 };
 
 use eyre::bail;
@@ -31,9 +31,41 @@ use tokio::sync::{
 
 // Learn more about Tauri commands at https://tauri.app/develop/calling-rust/
 
-type OutboundTransfers = Arc<
-    RwLock<std::collections::HashMap<String, Vec<(u64, tokio_util::sync::CancellationToken)>>>,
->;
+type OutboundTransfers =
+    Arc<RwLock<std::collections::HashMap<String, Vec<(u64, tokio_util::sync::CancellationToken)>>>>;
+
+const OUTBOUND_TRANSFER_EMIT_DEBOUNCE: Duration = Duration::from_millis(100);
+
+#[derive(Default)]
+struct OutboundTransferEmitState {
+    scheduled: bool,
+    generation: u64,
+}
+
+impl OutboundTransferEmitState {
+    fn record_change(&mut self) -> bool {
+        self.generation = self.generation.saturating_add(1);
+        if self.scheduled {
+            return false;
+        }
+
+        self.scheduled = true;
+        true
+    }
+
+    fn observed_generation(&self) -> u64 {
+        self.generation
+    }
+
+    fn finish_emit(&mut self, observed_generation: u64) -> bool {
+        if self.generation != observed_generation {
+            return true;
+        }
+
+        self.scheduled = false;
+        false
+    }
+}
 
 /// Tauri-managed runtime state shared by commands and setup tasks.
 #[derive(Default)]
@@ -48,6 +80,7 @@ struct LanSpreadState {
     unpack_logs: Arc<RwLock<Vec<UnpackLogEntry>>>,
     state_dir: OnceLock<PathBuf>,
     active_outbound_transfers: OutboundTransfers,
+    outbound_transfer_emit: Arc<RwLock<OutboundTransferEmitState>>,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]
@@ -1472,6 +1505,44 @@ fn spawn_peer_event_loop(app_handle: AppHandle, mut rx_peer_event: UnboundedRece
     });
 }
 
+async fn schedule_outbound_transfer_emit(app_handle: &AppHandle) {
+    let state = app_handle.state::<LanSpreadState>();
+    let should_spawn = {
+        let mut emit_state = state.outbound_transfer_emit.write().await;
+        emit_state.record_change()
+    };
+
+    if !should_spawn {
+        return;
+    }
+
+    let app_handle = app_handle.clone();
+    tauri::async_runtime::spawn(async move {
+        loop {
+            tokio::time::sleep(OUTBOUND_TRANSFER_EMIT_DEBOUNCE).await;
+
+            let observed_generation = {
+                let state = app_handle.state::<LanSpreadState>();
+                state
+                    .outbound_transfer_emit
+                    .read()
+                    .await
+                    .observed_generation()
+            };
+            emit_games_list(&app_handle).await;
+
+            let needs_follow_up_emit = {
+                let state = app_handle.state::<LanSpreadState>();
+                let mut emit_state = state.outbound_transfer_emit.write().await;
+                emit_state.finish_emit(observed_generation)
+            };
+            if !needs_follow_up_emit {
+                break;
+            }
+        }
+    });
+}
+
 #[allow(clippy::too_many_lines)]
 async fn handle_peer_event(app_handle: &AppHandle, event: PeerEvent) {
     match event {
@@ -1500,7 +1571,7 @@ async fn handle_peer_event(app_handle: &AppHandle, event: PeerEvent) {
         }
         PeerEvent::OutboundTransferCountChanged => {
             log::info!("PeerEvent::OutboundTransferCountChanged received");
-            emit_games_list(app_handle).await;
+            schedule_outbound_transfer_emit(app_handle).await;
         }
         PeerEvent::GotGameFiles {
             id,
@@ -1935,6 +2006,32 @@ mod tests {
         );
     }
 
+    #[test]
+    fn outbound_transfer_emit_state_coalesces_bursts_without_losing_updates() {
+        let mut state = OutboundTransferEmitState::default();
+
+        assert!(
+            state.record_change(),
+            "first change should schedule an emit"
+        );
+        assert_eq!(state.observed_generation(), 1);
+        assert!(
+            !state.record_change(),
+            "second change should reuse the scheduled emit"
+        );
+        assert_eq!(state.observed_generation(), 2);
+
+        assert!(
+            state.finish_emit(1),
+            "a generation observed before the latest change needs a follow-up emit"
+        );
+        assert!(
+            !state.finish_emit(2),
+            "the latest observed generation clears the scheduled emit"
+        );
+        assert!(state.record_change(), "a later burst should schedule again");
+    }
+
     #[test]
     fn game_file_viewer_ids_must_be_single_path_components() {
         assert!(is_single_component_game_id("game"));