Collect garbage between the phases of pnpm bench

bin/benchmark/run.mjs

@@ -177,6 +177,10 @@ async function bootAndRun({ headless = true } = {}) {
       // as the contradictory flags cause use-after-free crashes on macOS
       '--disable-gpu',
       '--disable-gpu-compositing',
+      // Exposes gc(), which the benchmark app calls between phases (see
+      // waitForIdle in smoke-tests/benchmark-app/app/run-benchmark.js), so
+      // the garbage of one phase is not collected inside a later one.
+      '--js-flags=--expose-gc',
     ].join(','),
   ];
 

smoke-tests/benchmark-app/app/run-benchmark.js

@@ -29,7 +29,16 @@ export function emitDomClickEvent(selector) {
 
 export function waitForIdle() {
   return new Promise((resolve) => {
-    requestIdleCallback(resolve);
+    requestIdleCallback(() => {
+      // Collect the previous phase's garbage outside of any measured phase.
+      // Chrome only exposes gc() with --js-flags=--expose-gc.
+      if (typeof globalThis.gc === 'function') {
+        globalThis.gc();
+        requestIdleCallback(resolve);
+      } else {
+        resolve();
+      }
+    });
   });
 }