fix(lint): declare updateDisplayPreview in ESLint global comment

Codacy flagged 'updateDisplayPreview is not defined' at app.js:73.
The function is defined in base.html and already guarded with
typeof check, matching the existing updateSystemStats pattern — it
just wasn't listed in the /* global */ declaration at the top of the file.

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

web_interface/app.py

@@ -2,8 +2,10 @@ from flask import Flask, request, redirect, url_for, jsonify, Response, send_fro
 import json
 import logging
 import os
+import queue
 import sys
 import subprocess
+import threading
 import time
 from pathlib import Path
 from datetime import datetime, timedelta
@@ -413,13 +415,53 @@ def add_security_headers(response):
     
     return response
 
-# SSE helper function
-def sse_response(generator_func):
-    """Helper to create SSE responses"""
-    def generate():
-        for data in generator_func():
-            yield f"data: {json.dumps(data)}\n\n"
-    return Response(generate(), mimetype='text/event-stream')
+class _StreamBroadcaster:
+    """Fan-out broadcaster: one background generator thread pushes to all SSE clients.
+
+    This means N browser tabs share one generator instead of each running their own,
+    keeping PIL encodes / subprocess forks constant regardless of how many tabs are open.
+    """
+
+    def __init__(self, generator_factory):
+        self._generator_factory = generator_factory
+        self._clients: set = set()
+        self._lock = threading.Lock()
+        self._thread: threading.Thread | None = None
+
+    def subscribe(self) -> queue.Queue:
+        q: queue.Queue = queue.Queue(maxsize=5)
+        with self._lock:
+            self._clients.add(q)
+            if not (self._thread and self._thread.is_alive()):
+                self._thread = threading.Thread(target=self._broadcast, daemon=True)
+                self._thread.start()
+        return q
+
+    def unsubscribe(self, q: queue.Queue) -> None:
+        with self._lock:
+            self._clients.discard(q)
+
+    def _broadcast(self):
+        for data in self._generator_factory():
+            with self._lock:
+                if not self._clients:
+                    # No subscribers — exit so the thread doesn't spin indefinitely.
+                    # subscribe() will restart it when a new client arrives.
+                    break
+                for q in self._clients:
+                    try:
+                        q.put_nowait(data)
+                    except queue.Full:
+                        # Client is reading too slowly; drop the oldest item and
+                        # 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():
@@ -596,20 +638,50 @@ def logs_generator():
 
         time.sleep(5)  # Update every 5 seconds (reduced frequency for better performance)
 
+# One broadcaster per stream — shared across all SSE clients
+_stats_broadcaster = _StreamBroadcaster(system_status_generator)
+_display_broadcaster = _StreamBroadcaster(display_preview_generator)
+_logs_broadcaster = _StreamBroadcaster(logs_generator)
+
+
+def _sse_stream(broadcaster: _StreamBroadcaster) -> Response:
+    """Return a streaming SSE response backed by a shared broadcaster."""
+    q = broadcaster.subscribe()
+
+    def generate():
+        try:
+            while True:
+                try:
+                    data = q.get(timeout=30)
+                    yield f"data: {json.dumps(data)}\n\n"
+                except queue.Empty:
+                    # Send an SSE comment heartbeat to keep the connection alive
+                    # through proxies that close idle connections.
+                    yield ": heartbeat\n\n"
+        except GeneratorExit:
+            pass
+        finally:
+            broadcaster.unsubscribe(q)
+
+    return Response(generate(), mimetype='text/event-stream')
+
+
 # SSE endpoints
 @app.route('/api/v3/stream/stats')
 def stream_stats():
-    return sse_response(system_status_generator)
+    return _sse_stream(_stats_broadcaster)
 
 @app.route('/api/v3/stream/display')
 def stream_display():
-    return sse_response(display_preview_generator)
+    return _sse_stream(_display_broadcaster)
 
 @app.route('/api/v3/stream/logs')
 def stream_logs():
-    return sse_response(logs_generator)
+    return _sse_stream(_logs_broadcaster)
 
-# Exempt SSE streams from CSRF and add rate limiting
+# Exempt SSE streams from CSRF and apply a generous rate limit.
+# SSE connections are long-lived HTTP requests, not repeated API calls, so the
+# tight "20 per minute" default would be exhausted quickly on reconnects.
 if csrf:
     csrf.exempt(stream_stats)
     csrf.exempt(stream_display)
@@ -617,9 +689,9 @@ if csrf:
     # Note: api_v3 blueprint is exempted above after registration
 
 if limiter:
-    limiter.limit("20 per minute")(stream_stats)
-    limiter.limit("20 per minute")(stream_display)
-    limiter.limit("20 per minute")(stream_logs)
+    limiter.limit("200 per minute")(stream_stats)
+    limiter.limit("200 per minute")(stream_display)
+    limiter.limit("200 per minute")(stream_logs)
 
 # Main route - redirect to v3 interface as default
 @app.route('/')

web_interface/static/v3/app.js

@@ -1,4 +1,4 @@
-/* global showNotification, updateSystemStats, htmx */
+/* global showNotification, updateSystemStats, updateDisplayPreview, htmx */
 // LED Matrix v3 JavaScript
 // Additional helpers for HTMX and Alpine.js integration
 
@@ -51,7 +51,8 @@ document.body.addEventListener('htmx:afterRequest', function(event) {
     }
 });
 
-// SSE reconnection helper
+// 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();
@@ -60,14 +61,18 @@ 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) {
         window.displaySource.close();
         window.displaySource = new EventSource('/api/v3/stream/display');
-        window.displaySource.onmessage = function() {
-            // Handle display updates
+        window.displaySource.onmessage = function(event) {
+            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

@@ -1370,34 +1370,64 @@
 
     <!-- SSE connection for real-time updates -->
     <script>
-        // Connect to SSE streams
-        const statsSource = new EventSource('/api/v3/stream/stats');
-        const displaySource = new EventSource('/api/v3/stream/display');
+        // Assign to window so reconnectSSE() in app.js can reach them.
+        window.statsSource = new EventSource('/api/v3/stream/stats');
+        window.displaySource = new EventSource('/api/v3/stream/display');
 
-        statsSource.onmessage = function(event) {
+        window.statsSource.onmessage = function(event) {
             const data = JSON.parse(event.data);
             updateSystemStats(data);
         };
 
-        displaySource.onmessage = function(event) {
+        window.displaySource.onmessage = function(event) {
             const data = JSON.parse(event.data);
             updateDisplayPreview(data);
         };
 
-        // Connection status
-        statsSource.addEventListener('open', function() {
-            document.getElementById('connection-status').innerHTML = `
+        function _setConnectionStatus(connected, reconnecting) {
+            const el = document.getElementById('connection-status');
+            if (!el) return;
+            if (connected) {
+                el.innerHTML = `
                     <div class="w-2 h-2 bg-green-500 rounded-full"></div>
                     <span class="text-gray-600">Connected</span>
                 `;
-        });
-
-        statsSource.addEventListener('error', function() {
-            document.getElementById('connection-status').innerHTML = `
-                <div class="w-2 h-2 bg-red-500 rounded-full"></div>
-                <span class="text-gray-600">Disconnected</span>
+            } else if (reconnecting) {
+                el.innerHTML = `
+                    <div class="w-2 h-2 bg-yellow-500 rounded-full animate-pulse"></div>
+                    <span class="text-gray-600">Reconnecting…</span>
                 `;
-        });
+            } else {
+                el.innerHTML = `
+                    <div class="w-2 h-2 bg-red-500 rounded-full"></div>
+                    <span class="text-gray-600" title="Connection lost — try refreshing the page">Disconnected</span>
+                `;
+            }
+        }
+
+        var _statsErrorCount = 0;
+
+        // 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() {
+            _statsErrorCount++;
+            // EventSource readyState 0 = CONNECTING (auto-retrying), 2 = CLOSED
+            var reconnecting = window.statsSource.readyState === EventSource.CONNECTING;
+            _setConnectionStatus(false, reconnecting && _statsErrorCount <= 3);
+        };
+        window._displayErrorHandler = 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