Bugfix: MTP event debouncer drops events

vdavid · vdavid · commit 21b3bc5f1821 · 2026-04-03T15:03:16.000+02:00
Now ALL MTP E2E tests are passing! This is a milestone! We have reliable MTP, and reliable MTP E2E tests!

- `emit_directory_changed` permanently dropped events suppressed by the 500ms debounce window. Now schedules a trailing emit after the window expires, so the last event in a burst always gets processed.
- Formatting fix in `watcher.rs` (indentation only).
diff --git a/apps/desktop/src-tauri/src/file_system/watcher.rs b/apps/desktop/src-tauri/src/file_system/watcher.rs
@@ -356,14 +356,15 @@ pub fn handle_directory_change(listing_id: &str) {
         use crate::file_system::listing::caching::LISTING_CACHE;
         if let Ok(cache) = LISTING_CACHE.read()
             && let Some(listing) = cache.get(listing_id)
-                && let Some(vol) = crate::file_system::get_volume_manager().get(&listing.volume_id)
-                    && !vol.supports_watching() {
-                        log::debug!(
-                            "handle_directory_change: skipping non-watchable volume (volume={})",
-                            listing.volume_id
-                        );
-                        return;
-                    }
+            && let Some(vol) = crate::file_system::get_volume_manager().get(&listing.volume_id)
+            && !vol.supports_watching()
+        {
+            log::debug!(
+                "handle_directory_change: skipping non-watchable volume (volume={})",
+                listing.volume_id
+            );
+            return;
+        }
     }
 
     // Get old entries and path from the unified LISTING_CACHE
diff --git a/apps/desktop/src-tauri/src/mtp/connection/event_loop.rs b/apps/desktop/src-tauri/src/mtp/connection/event_loop.rs
@@ -155,12 +155,22 @@ impl MtpConnectionManager {
     /// Uses the unified diff system shared with local file watching, providing
     /// smooth incremental UI updates without full directory reloads.
     fn emit_directory_changed(device_id: &str, app: &AppHandle) {
-        // Check debouncer via the global connection manager
+        // Check debouncer via the global connection manager.
+        // When suppressed, schedule a trailing emit after the debounce window
+        // so the last event in a burst is never permanently dropped.
         if !connection_manager().event_debouncer.should_emit(device_id) {
             debug!(
-                "MTP event loop: directory change DEBOUNCED for device={} (within {}ms window)",
+                "MTP event loop: directory change DEBOUNCED for device={} (within {}ms window), scheduling trailing emit",
                 device_id, EVENT_DEBOUNCE_MS
             );
+            let device_id_owned = device_id.to_string();
+            let app_clone = app.clone();
+            tokio::spawn(async move {
+                tokio::time::sleep(Duration::from_millis(EVENT_DEBOUNCE_MS + 50)).await;
+                // Re-emit — this goes through the debouncer again (which will pass
+                // since the window has expired) to avoid duplicate processing.
+                Self::emit_directory_changed(&device_id_owned, &app_clone);
+            });
             return;
         }
 
