fix: ensure stable testid on baseElement and container

Port upstream fix from vitest-browser-react (vitest-community#42, vitest-community#44, vitest-community#45) to prevent
flaky locators when baseElement/container lack a testid attribute.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ShaulLavo

src/pure.tsx

@@ -3,7 +3,7 @@ import type {
 	LocatorSelectors,
 	PrettyDOMOptions,
 } from 'vitest/browser'
-import { page, utils } from 'vitest/browser'
+import { page, server, utils } from 'vitest/browser'
 import {
 	createRoot,
 	getOwner,
@@ -16,6 +16,14 @@ import type { JSX } from 'solid-js'
 
 const { debug, getElementLocatorSelectors } = utils
 
+let idx = 0
+function ensureTestIdAttribute(element: HTMLElement) {
+	const attributeId = server.config.browser.locators.testIdAttribute
+	if (!element.hasAttribute(attributeId)) {
+		element.setAttribute(attributeId, `__vitest_${idx++}__`)
+	}
+}
+
 export interface RenderResult extends LocatorSelectors {
 	container: HTMLElement
 	baseElement: HTMLElement
@@ -56,6 +64,11 @@ export function render(
 		container = baseElement.appendChild(document.createElement('div'))
 	}
 
+	// Ensuring testid attributes exist so that the generated locators will be stable
+	// https://github.com/vitest-community/vitest-browser-react/issues/42
+	ensureTestIdAttribute(baseElement)
+	ensureTestIdAttribute(container)
+
 	// Dispose previous root if it exists on the same container
 	const prevDispose = mountedRoots.get(container)
 	if (prevDispose) {