test: scope autoplay flag to Chromium

tests/e2e/playwright.config.mjs

@@ -17,19 +17,20 @@ export default defineConfig({
     expect: { timeout: 10_000 },
     use: {
         baseURL: `http://localhost:${PORT}`,
-        trace: 'retain-on-failure',
+        trace: 'retain-on-failure'
-        launchOptions: {
-            // Several fixtures hinge on a video actually playing. Without this
-            // the browser's autoplay heuristics decide whether the fixture is
-            // valid, which shows up later as an unexplained flake.
-            args: ['--autoplay-policy=no-user-gesture-required']
-        }
     },
     projects: [
         {
             name: 'detection-chromium',
             testMatch: 'detection.spec.mjs',
-            use: { browserName: 'chromium' }
+            use: {
+                browserName: 'chromium',
+                launchOptions: {
+                    // Chromium alone supports this switch. Passing it through
+                    // the shared config makes Linux WebKit refuse to launch.
+                    args: ['--autoplay-policy=no-user-gesture-required']
+                }
+            }
         },
         {
             name: 'detection-firefox',