FN-5876: prevent mobile quick-entry autofocus

Avoid restoring focus to the new-task quick entry on mobile while preserving desktop behavior.

- gate quick-entry autofocus and post-submit focus restoration behind a desktop-width check
- add desktop and mobile coverage for initial focus behavior
- add mobile coverage for avoiding focus restoration after successful task creation

Files changed:
 .../dashboard/app/components/QuickEntryBox.tsx     |  5 ++-
 .../components/__tests__/QuickEntryBox.test.tsx    | 42 ++++++++++++++++++++++
 2 files changed, 46 insertions(+), 1 deletion(-)

Fusion-Task-Id: FN-5876

Fusion-Task-Lineage: ede08207-6506-4f5e-8a6f-f7f1cbfb37c9

packages/dashboard/app/components/QuickEntryBox.tsx

@@ -16,6 +16,7 @@ import { NodeHealthDot } from "./NodeHealthDot";
 import { ProviderIcon } from "./ProviderIcon";
 
 const STORAGE_KEY = "kb-quick-entry-text";
+const MOBILE_BREAKPOINT_PX = 768;
 const ALLOWED_IMAGE_TYPES = ["image/png", "image/jpeg", "image/gif", "image/webp"];
 
 interface PendingImage {
@@ -329,7 +330,9 @@ export function QuickEntryBox({ onCreate, addToast, tasks = [], availableModels,
     if (!isSubmitting && description === "" && textareaRef.current) {
       // Use setTimeout to ensure focus happens after React re-enables the textarea
       const focusTimeout = setTimeout(() => {
-        textareaRef.current?.focus();
+        if (typeof window !== "undefined" && window.innerWidth > MOBILE_BREAKPOINT_PX) {
+          textareaRef.current?.focus();
+        }
       }, 0);
       return () => clearTimeout(focusTimeout);
     }

packages/dashboard/app/components/__tests__/QuickEntryBox.test.tsx

@@ -295,6 +295,28 @@ describe("QuickEntryBox", () => {
     expect((textarea as HTMLTextAreaElement).rows).toBe(2);
   });
 
+  it("focuses the quick-entry textarea on mount at desktop width", async () => {
+    mockDesktopViewport();
+    renderQuickEntryBox({});
+    const textarea = screen.getByTestId("quick-entry-input");
+
+    await waitFor(() => {
+      expect(document.activeElement).toBe(textarea);
+    });
+  });
+
+  it("does not focus the quick-entry textarea on mount at mobile width", async () => {
+    const innerWidthSpy = vi.spyOn(window, "innerWidth", "get").mockReturnValue(375);
+    renderQuickEntryBox({});
+    const textarea = screen.getByTestId("quick-entry-input");
+
+    await waitFor(() => {
+      expect(document.activeElement).not.toBe(textarea);
+    });
+
+    innerWidthSpy.mockRestore();
+  });
+
   it("textarea spans full container width (FN-1608)", () => {
     mockDesktopViewport();
     renderQuickEntryBox({});
@@ -643,6 +665,26 @@ describe("QuickEntryBox", () => {
     });
   });
 
+  it("does not restore focus after successful creation at mobile width", async () => {
+    const innerWidthSpy = vi.spyOn(window, "innerWidth", "get").mockReturnValue(375);
+    const { props } = renderQuickEntryBox({});
+    const textarea = screen.getByTestId("quick-entry-input");
+
+    fireEvent.focus(textarea);
+    fireEvent.change(textarea, { target: { value: "Task to create" } });
+    fireEvent.keyDown(textarea, { key: "Enter" });
+
+    await waitFor(() => {
+      expect(props.onCreate).toHaveBeenCalled();
+    });
+
+    await waitFor(() => {
+      expect(document.activeElement).not.toBe(textarea);
+    });
+
+    innerWidthSpy.mockRestore();
+  });
+
   describe("Rich creation features", () => {
     it("shows inline deps/models/save controls when expanded", () => {
       renderQuickEntryBox({});