Fix chat UI stuck loading after first message send failure (microsoft#2110)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: romanlutz

frontend/e2e/errors.spec.ts

@@ -264,6 +264,29 @@ test.describe("Error: backend 500 on send message", () => {
     // The failed text should be restored in the input for easy re-send
     await expect(input).toHaveValue("This should fail", { timeout: 5000 });
   });
+
+  test("should recover cleanly when the first send fails", async ({ page }) => {
+    await mockAllAPIs(page, async (route) => {
+      await route.fulfill({
+        status: 500,
+        contentType: "application/json",
+        body: JSON.stringify({ detail: "Internal server error" }),
+      });
+    });
+
+    await page.goto("/");
+    await activateMockTarget(page);
+
+    const input = page.getByRole("textbox");
+    await input.fill("First send fails");
+    await page.getByRole("button", { name: /send/i }).click();
+
+    await expect(page.getByText(/Internal server error/i)).toBeVisible({
+      timeout: 10000,
+    });
+    await expect(page.getByTestId("loading-state")).toHaveCount(0);
+    await expect(input).toHaveValue("First send fails", { timeout: 5000 });
+  });
 });
 
 // ---------------------------------------------------------------------------

frontend/src/components/Chat/ChatWindow.tsx

@@ -347,8 +347,21 @@ export default function ChatWindow({
         setLoadedConversationId(effectiveConvId!)
       }
     } catch (err) {
+      const viewedConversationId = viewedConvRef.current
+      const isViewingFailedConversation = viewedConversationId === sendConvId
+        || viewedConversationId === (activeConversationId ?? conversationId)
+        || (viewedConversationId == null && sendConvId !== '__pending__')
+
       // Only show error in UI if user is still on this conversation
-      if (viewedConvRef.current === sendConvId || viewedConvRef.current === (activeConversationId ?? conversationId)) {
+      if (isViewingFailedConversation) {
+        // Mark the viewed conversation as loaded so first-send failures do not
+        // get stuck behind the "Loading conversation..." placeholder.
+        if (viewedConversationId) {
+          setLoadedConversationId(viewedConversationId)
+        } else if (sendConvId !== '__pending__') {
+          setLoadedConversationId(sendConvId)
+        }
+
         const apiError = toApiError(err)
         let description: string
         if (apiError.isNetworkError) {