Close SSE connection nicely on navigating away from the page

webassets/js/spots.js

@@ -4,6 +4,14 @@ let restartSSEOnErrorTimeoutId;
 // Table row count, to alternate shading
 let rowCount = 0;
 
+// Set up a listener to close the SSE connection nicely when we navigate away from the page, to prevent console errors
+// and keep things nice and tidy for the server.
+window.addEventListener('beforeunload', function() {
+    if (evtSource != null) {
+        evtSource.close();
+    }
+});
+
 // Load spots and populate the table.
 function loadSpots() {
     // If we have an ongoing SSE connection, stop it so it doesn't interfere with our reload