Load each bar widget's component once

Qt.createComponent is asynchronous, but a widget was only recorded in
pluginWidgetComponents once its component finished. syncPluginWidgets runs
several times while the shell starts, and every pass that ran while a load
was still in flight could not tell it apart from one that had never
happened — so it started a second load of the same URL. All twenty bar
widgets were loaded twice.

The duplicate component then replaced the first in the registry, and
swapping a slot's component makes its Loader build the replacement before
dropping the original. Both were briefly live, and both registered the
widget's IPC handler, which is where the "another handler is registered
for target" warnings came from.

Claim the key when the load starts instead, and release it if the
component fails so a later rescan can retry.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

shell/shell.qml

@@ -700,6 +700,12 @@ ShellRoot {
         source: "plugin"
       }
 
+      // A load already in flight for this URL registers itself when it
+      // finishes. Starting a second one produces a second Component for the
+      // same widget, and swapping a slot's component rebuilds its item —
+      // briefly running two of the widget, each registering its IPC handler.
+      if (existing && existing.url === url && !existing.component) continue
+
       // If the component URL is unchanged, just refresh the metadata in
       // place. We can't skip this even when the URL matches: manifests can
       // change schema, defaults, or sourceDir between rescans, and the
@@ -769,17 +775,29 @@ ShellRoot {
     }
   }
 
+  function setPluginWidgetComponent(registryKey, entry) {
+    var next = ({})
+    for (var k in pluginWidgetComponents) if (k !== registryKey) next[k] = pluginWidgetComponents[k]
+    if (entry) next[registryKey] = entry
+    pluginWidgetComponents = next
+  }
+
   function loadPluginWidget(registryKey, url, meta) {
+    // Claim the key before the component exists. Qt.createComponent is
+    // asynchronous and syncPluginWidgets runs several times while the shell
+    // starts, so without a marker the later passes cannot tell a load in
+    // flight from one that never happened.
+    setPluginWidgetComponent(registryKey, { url: url, component: null })
+
     var comp = Qt.createComponent(url, Component.Asynchronous)
     function finalize() {
       if (comp.status === Component.Ready) {
         shell.barWidgetRegistry.register(registryKey, comp, meta)
-        var next = ({})
-        for (var k in pluginWidgetComponents) next[k] = pluginWidgetComponents[k]
-        next[registryKey] = { url: url, component: comp }
-        pluginWidgetComponents = next
+        shell.setPluginWidgetComponent(registryKey, { url: url, component: comp })
       } else if (comp.status === Component.Error) {
         console.warn("Plugin widget " + registryKey + " failed: " + comp.errorString())
+        // Drop the claim so a later rescan can retry.
+        shell.setPluginWidgetComponent(registryKey, null)
         shell.pluginRegistry.pluginLoadFailed(registryKey, comp.errorString())
       }
     }