Why does Promise.all wait for slow microtasks even if all Promises resolve immediately? [duplicate]

Note: This is not a duplicate of questions about blocking or long-running loops. There is no CPU-intensive or infinite loop here — the issue is that Promise.all is unexpectedly delayed due to unrelated queueMicrotask() calls. It’s a subtle event loop/microtask interaction, not a case of blocking code.

I’m working with some async code where all Promises are already resolved, but there’s also a queue of microtasks triggered before Promise.all runs. Surprisingly, Promise.all still seems to wait for all of them to finish before resolving, even though the Promises themselves are resolved instantly.

Here’s a minimal repro:

 // This is a synchronous busy loop that blocks the event loop
function busyLoop(iterations) {
  const start = Date.now();
  while (Date.now() - start < iterations) {
    // intentionally blocking
  }
}

function microtaskFlood() {
  for (let i = 0; i < 1000; i++) {
    queueMicrotask(() => {
      // Just a microtask that does nothing
    });
  }
}

async function test() {
  console.log('Start');

  busyLoop(100); // Blocks event loop synchronously for ~100 ms

  // Schedule many microtasks after blocking
  microtaskFlood();

  // Promise.all waits for these microtasks to finish
  await Promise.all([
    Promise.resolve('done1'),
    Promise.resolve('done2')
  ]);

  console.log('End');
}

test();

Expected:
Since both a and b are resolved, I expected Promise.all to resolve immediately.

Actual:
The Promise.all .then() only fires after the microtask finishes. This seems unintuitive because the microtask has no relation to a or b.

Questions:

1. Is this behavior according to the ECMAScript spec?
2. Why doesn’t Promise.all resolve in the same tick when all Promises are already resolved?
3. Is there a way to make it resolve without waiting on unrelated microtasks?

Any explanation about how JavaScript’s microtask queue affects Promise.all scheduling would help.

Thanks!