[fix](trx-frontend): serialize plugin loading

src/trx-client/trx-frontend/trx-frontend-http/assets/web/index.html

@@ -1641,29 +1641,56 @@ SPDX-License-Identifier: GPL-2.0-or-later
     'settings': ['/vchan.js', '/scheduler.js']
   };
   var loaded = new Set();
-  function loadPlugins(tab) {
-    var scripts = pluginScripts[tab];
-    if (!scripts) return;
-    scripts.forEach(function(src) {
-      if (loaded.has(src)) return;
-      loaded.add(src);
+  var loading = new Map();
+
+  function loadScript(src) {
+    if (loaded.has(src)) return Promise.resolve();
+    if (loading.has(src)) return loading.get(src);
+
+    var request = new Promise(function(resolve, reject) {
       var s = document.createElement('script');
       s.src = src;
-      s.defer = true;
+      s.onload = function() {
+        loaded.add(src);
+        loading.delete(src);
+        resolve();
+      };
+      s.onerror = function() {
+        loading.delete(src);
+        reject(new Error('Failed to load plugin script: ' + src));
+      };
       document.body.appendChild(s);
     });
+    loading.set(src, request);
+    return request;
+  }
+
+  function loadPlugins(tab) {
+    var scripts = pluginScripts[tab];
+    if (!scripts) return Promise.resolve();
+    return scripts.reduce(function(sequence, src) {
+      return sequence.then(function() { return loadScript(src); });
+    }, Promise.resolve());
+  }
+
+  function requestPlugins(tab) {
+    return loadPlugins(tab).catch(function(err) {
+      console.error(err);
+    });
   }
   // Eager plugin loading is triggered by app.js (after window.trx is set up)
   // via window.loadEagerPlugins().  Dynamic scripts are effectively async, so
   // loading them before app.js would cause map-core.js to crash when
   // window.trx is not yet defined.
   window.loadEagerPlugins = function() {
-    ['digital-modes', 'map-data', 'bookmarks', 'settings'].forEach(loadPlugins);
+    return Promise.all(
+      ['digital-modes', 'map-data', 'bookmarks', 'settings'].map(requestPlugins)
+    );
   };
   // Load others on tab switch
   document.addEventListener('click', function(e) {
     var tab = e.target.closest('[data-tab]');
-    if (tab) loadPlugins(tab.dataset.tab);
+    if (tab) requestPlugins(tab.dataset.tab);
   });
   window.loadPluginsForTab = loadPlugins;
 })();