fix(web-ui): harden SSE broadcaster — drop-oldest on full queue, exit on no subscribers, reattach reconnect handlers

- _broadcast: on queue.Full drop the oldest item and retry the put
  instead of removing the client from _clients — a slow tab now stays
  subscribed and receives the latest data rather than being silently
  ejected
- _broadcast: break instead of continue when _clients is empty so the
  background generator thread exits rather than spinning indefinitely;
  subscribe() already restarts it on the next connection
- base.html: expose _statsOpenHandler, _statsErrorHandler, and
  _displayErrorHandler as window properties so reconnectSSE() can
  reattach them after replacing the EventSource instances
- app.js: reconnectSSE() now reattaches those handlers after creating
  each new EventSource so the status badge and display-stream console
  logging survive a manual reconnect

Heartbeat path (~line 646) is a queue read (q.get), not a write; no
queue.Full can occur there so no change needed.

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

web_interface/app.py

@@ -445,14 +445,23 @@ class _StreamBroadcaster:
         for data in self._generator_factory():
             with self._lock:
                 if not self._clients:
-                    continue
+                    # No subscribers — exit so the thread doesn't spin indefinitely.
-                dead = set()
+                    # subscribe() will restart it when a new client arrives.
+                    break
                 for q in self._clients:
                     try:
                         q.put_nowait(data)
                     except queue.Full:
-                        dead.add(q)
+                        # Client is reading too slowly; drop the oldest item and
-                self._clients -= dead
+                        # deliver the latest so the queue never stalls the client.
+                        try:
+                            q.get_nowait()
+                        except queue.Empty:
+                            pass
+                        try:
+                            q.put_nowait(data)
+                        except queue.Full:
+                            pass
 
 # System status generator for SSE
 def system_status_generator():

web_interface/static/v3/app.js

@@ -51,7 +51,8 @@ document.body.addEventListener('htmx:afterRequest', function(event) {
     }
 });
 
-// SSE reconnection helper — closes and reopens both SSE streams.
+// SSE reconnection helper — closes and reopens both SSE streams,
+// reattaching the open/error handlers defined in base.html.
 window.reconnectSSE = function() {
     if (window.statsSource) {
         window.statsSource.close();
@@ -60,6 +61,8 @@ window.reconnectSSE = function() {
             const data = JSON.parse(event.data);
             if (typeof updateSystemStats === 'function') updateSystemStats(data);
         };
+        if (window._statsOpenHandler) window.statsSource.addEventListener('open', window._statsOpenHandler);
+        if (window._statsErrorHandler) window.statsSource.addEventListener('error', window._statsErrorHandler);
     }
 
     if (window.displaySource) {
@@ -69,6 +72,7 @@ window.reconnectSSE = function() {
             const data = JSON.parse(event.data);
             if (typeof updateDisplayPreview === 'function') updateDisplayPreview(data);
         };
+        if (window._displayErrorHandler) window.displaySource.addEventListener('error', window._displayErrorHandler);
     }
 };
 

web_interface/templates/v3/base.html

@@ -1406,23 +1406,28 @@
         }
 
         var _statsErrorCount = 0;
-        window.statsSource.addEventListener('open', function() {
+
+        // Named on window so reconnectSSE() in app.js can reattach them after
+        // replacing the EventSource instances.
+        window._statsOpenHandler = function() {
             _statsErrorCount = 0;
             _setConnectionStatus(true, false);
-        });
+        };
-
+        window._statsErrorHandler = function() {
-        window.statsSource.addEventListener('error', function() {
             _statsErrorCount++;
             // EventSource readyState 0 = CONNECTING (auto-retrying), 2 = CLOSED
             var reconnecting = window.statsSource.readyState === EventSource.CONNECTING;
             _setConnectionStatus(false, reconnecting && _statsErrorCount <= 3);
-        });
+        };
-
+        window._displayErrorHandler = function() {
-        window.displaySource.addEventListener('error', function() {
             // Display stream errors don't change the status badge but log to console
             // so failures aren't completely silent.
             console.warn('LEDMatrix: display preview stream error (readyState=' + window.displaySource.readyState + ')');
-        });
+        };
+
+        window.statsSource.addEventListener('open', window._statsOpenHandler);
+        window.statsSource.addEventListener('error', window._statsErrorHandler);
+        window.displaySource.addEventListener('error', window._displayErrorHandler);
 
         function updateSystemStats(data) {
             // Update CPU in header