Put the provider/model picker above the dialog that opens it

The picker (node-settings-dialog) shared --z-modal with every other dialog
guest, so opening it from "Evaluate impact with LLM" - itself a modal - tied
on z-index with the report behind it and lost on DOM order: it rendered, and
its backdrop even blocked clicks, but neither was visible. It read as a
button that did nothing.

Named the layer this actually is - --z-dialog-over-modal, the same one the
confirmation dialog already needed and had defined ad hoc as --z-confirm -
and moved the picker onto it.

While chasing this, closed a real silence next to it: with no LLM provider
published at all, the picker answered null and the caller treated that like
a dismissal, so the button did nothing for a second, unrelated reason. It now
throws with a message, and Simulate surfaces it as a notification instead of
swallowing it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/shared/llm-descriptor-settings/llm-descriptor-settings.spec.ts

@@ -72,10 +72,17 @@ describe('openLLMDescriptorSettings', () => {
     expect(open.mock.calls[0][0].initial.seed).toBeUndefined();
   });
 
-  it('answers null when no provider is published at all', async () => {
+  it('says so, instead of opening nothing, when no provider is published at all', async () => {
     const { service, open } = dialog({ provider: 'x', model: 'y' });
 
-    expect(await openLLMDescriptorSettings(service, retriever([]), { title: 'Simulation Settings' })).toBeNull();
+    await expect(openLLMDescriptorSettings(service, retriever([]), { title: 'Simulation Settings' }))
+      .rejects.toThrow('No LLM provider is published');
     expect(open).not.toHaveBeenCalled();
   });
+
+  it('answers null when the dialog is dismissed', async () => {
+    const { service } = dialog(null);
+
+    expect(await openLLMDescriptorSettings(service, retriever(), { title: 'Simulation Settings' })).toBeNull();
+  });
 });

src/app/shared/llm-descriptor-settings/llm-descriptor-settings.ts

@@ -48,7 +48,8 @@ export type LLMDescriptorSettingsRequest = {
  * They ask the same question, so they ask it with the same window - the provider list, the models
  * that follow from the chosen provider, and the same optional parameters.
  *
- * Returns null when the dialog is dismissed, or when no provider is published at all.
+ * Returns null when the dialog is dismissed. Throws when no provider is published at all: the
+ * caller has a place to say so, and answering null there made the button that opened it look broken.
  */
 export async function openLLMDescriptorSettings(
   settingsDialog: NodeSettingsDialogService,
@@ -57,7 +58,7 @@ export async function openLLMDescriptorSettings(
 ): Promise<LLMDescriptor | null> {
   const providerOptions = await loadOptions(fieldRetriever, 'providers', {}, PROVIDER_RETRIEVER_URL);
   if (!providerOptions.length) {
-    return null;
+    throw new Error('No LLM provider is published by the server, so there is no model to choose.');
   }
 
   const inherited = request.initialDescriptor ?? null;

src/app/shared/node-settings-dialog/node-settings-dialog.html

@@ -1,5 +1,5 @@
 @if (state()) {
-<div class="fixed inset-0 z-(--z-modal)">
+<div class="fixed inset-0 z-(--z-dialog-over-modal)">
   <div class="absolute inset-0 bg-black/50" (click)="cancel($event)"></div>
 
   <div class="absolute left-1/2 top-1/2 flex max-h-[min(80vh,720px)] w-[min(92vw,560px)] -translate-x-1/2 -translate-y-1/2 flex-col gap-4 overflow-hidden rounded-2xl border border-(--color-border) bg-white p-6 shadow-(--shadow-modal)">

src/app/shared/task-execution-viewer/task-execution-viewer.html

@@ -527,7 +527,9 @@
     [executionId]="execution()!.id"
     [annotatedNodeCount]="biasAnnotatedNodeCount()"
     [blockedReason]="biasExperimentBlockedReason()"
-    (startExperimentRequested)="openBiasedRerunDialog()" />
+    [comparableVariant]="canCompareBiasExecution()"
+    (startExperimentRequested)="openBiasedRerunDialog()"
+    (compareRequested)="openCompareDialog()" />
           }
         </div>
       </aside>

src/app/shared/task-execution-viewer/task-execution-viewer.ts

@@ -1132,7 +1132,19 @@ export class TaskExecutionViewerComponent implements OnDestroy {
     const executionId = this.execution()?.id;
     if (!executionId || !this.canSimulateExecution()) return;
 
-    const simulator = await this.openSimulationSettings();
+    let simulator: LLMDescriptor | null;
+    try {
+      simulator = await this.openSimulationSettings();
+    } catch (error) {
+      // No provider to choose from, or the list could not be loaded. Silence here reads as a dead
+      // button, which is what the picker is opened by.
+      this.notifications.show(
+        error instanceof Error ? error.message : 'Unable to load the providers to simulate with.',
+        'error',
+        6000
+      );
+      return;
+    }
     if (!simulator) return;
 
     this.simulateInProgress.set(true);

src/styles.css

@@ -65,7 +65,14 @@
   --space-5: 20px;
   --z-modal-backdrop: 10020;
   --z-modal: 10021;
-  --z-confirm: 10031;
+  /*
+   * Dialogs that are opened from inside another dialog: a confirmation, and the provider/model
+   * picker. At --z-modal they tied with the modal that opened them and lost on DOM order, so the
+   * picker for "Evaluate impact with LLM" opened behind the report it was asked for and read as a
+   * button that did nothing.
+   */
+  --z-dialog-over-modal: 10031;
+  --z-confirm: var(--z-dialog-over-modal);
 }
 
 /* ── Semantic surface colours (hard-coded) ──────────────────────