fix: refocus chat input after message send completes (#22666)

## Problem

The chat input loses focus after sending a message. Users have to click
back into the input field to type their next message.

## Root Cause

When a message is sent:
1. `handleSubmit` in `AgentChatInput` calls `onSend(text)` then
immediately `focus()` — but this is premature
2. The async send sets `isLoading=true`, which disables the editor via
`EditableStatePlugin`
3. When the send resolves, `handleSendFromInput` calls `clear()` and
`focus()` — but the editor may still be disabled at this point (React
hasn't re-rendered yet)
4. When React re-renders with `isLoading=false`, the editor becomes
editable again but nobody restores focus

## Fix

Added a `useEffect` in `AgentChatInput` that watches for `isLoading`
transitioning from `true` to `false` and calls `focus()` on the editor.
This ensures focus is restored *after* React has re-enabled the editor,
not prematurely.

## Test

Added a test in `AgentDetail.test.ts` verifying that `focus()` is called
on the input ref after `handleSendFromInput` resolves.

site/src/pages/AgentsPage/AgentChatInput.tsx

@@ -263,6 +263,18 @@ export const AgentChatInput = memo<AgentChatInputProps>(
 			[onContentChange],
 		);
 
+		// Re-focus the editor after a send completes (isLoading goes
+		// from true → false) so the user can immediately type again.
+		// Uses the "store previous value in state" pattern recommended
+		// by React for responding to prop changes during render.
+		const [prevIsLoading, setPrevIsLoading] = useState(isLoading);
+		if (prevIsLoading !== isLoading) {
+			setPrevIsLoading(isLoading);
+			if (prevIsLoading && !isLoading) {
+				internalRef.current?.focus();
+			}
+		}
+
 		const canSend = !isDisabled && !isLoading && hasModelOptions && hasContent;
 
 		const handleSubmit = useCallback(() => {

site/src/pages/AgentsPage/AgentDetail.test.ts

@@ -1,4 +1,5 @@
 import { act, renderHook } from "@testing-library/react";
+
 import { beforeEach, describe, expect, it, vi } from "vitest";
 import {
 	draftInputStorageKeyPrefix,
@@ -86,6 +87,33 @@ describe("useConversationEditingState", () => {
 		unmount();
 	});
 
+	it("calls focus on the input ref after a successful send", async () => {
+		const { result, onSend, unmount } = renderEditing();
+
+		// Attach a mock ChatMessageInputRef to the chatInputRef
+		const mockFocus = vi.fn();
+		const mockClear = vi.fn();
+		const mockInputRef = {
+			focus: mockFocus,
+			clear: mockClear,
+			insertText: vi.fn(),
+			getValue: vi.fn().mockReturnValue(""),
+		};
+		// The hook exposes chatInputRef – assign the mock to it.
+		result.current.chatInputRef.current = mockInputRef;
+
+		await act(async () => {
+			result.current.handleSendFromInput("hello");
+			await vi.waitFor(() => {
+				expect(onSend).toHaveBeenCalledWith("hello", undefined);
+			});
+		});
+
+		expect(mockClear).toHaveBeenCalled();
+		expect(mockFocus).toHaveBeenCalled();
+		unmount();
+	});
+
 	it("clears the draft from localStorage on successful send", async () => {
 		localStorage.setItem(expectedKey, "draft to clear");