diff --git a/packages/ui/vitest.config.ts b/packages/ui/vitest.config.ts
index 842ab894..20c02c6b 100644
--- a/packages/ui/vitest.config.ts
+++ b/packages/ui/vitest.config.ts
@@ -9,6 +9,11 @@ export default defineConfig({
 		include: ['src/**/*.{test,spec}.{js,ts}'],
 		exclude: ['src/**/*-ssr.test.ts'],
 		globals: true,
+		// One test file at a time. In parallel, each file runs in its own iframe, and the browser
+		// throttles the frames that are off screen and moves the focus between them: presence
+		// exits that wait for a frame stall, and a focus that lands on a trigger opens a tooltip
+		// nobody asked for. Serial runs cost a minute; the flakes cost a rerun each.
+		fileParallelism: false,
 		alias: {
 			$lib: resolve(__dirname, './src/lib'),
 			'$app/environment': resolve(__dirname, './src/lib/test-mocks/app-environment.ts'),