Change default async generator component behavior to pause at yields

[brainkim]

Summary

This issue proposes a breaking change to async generator components: their default behavior will be changed so that, when not in a props loop (for {} of this, for await {} of this), they pause at the first yield until rerendered, rather than continuously resuming. Currently, the default behavior is to resume as though the component is in a for await...of loop. This is a continuation of the effort started in #243 to align the behavior of async generator components with sync generator components to make them more predictable.

Motivation

• Ensures components respond to props and state changes.
• Discourage while (true) components.
• Reduces confusion for developers refactoring between sync and async generator components.
• Makes continuous polling/timer logic explicit and easier to reason about.
• Async generator components which use for...of and initial yields work exactly the same as sync generator components:

  async function *Component() {
    // currently, this yield resumes immediately,
    // but it would be better to behave like sync generator components
    yield initial;
    for ({} of this) {
      yield nonInitial;
    }
  }

• In 0.7, the result passed back into async generator components is always a promise, but if the sync gen behavior is used, we can pass back in the actual value because we block on children.

Breaking Change

• Components that previously relied on automatic continuous resuming (outside of loops) will now need explicit refresh calls or should use a for await...of loop for continuous updates.
• Migration guidance should be provided in release notes.
• Might be alleviated with the addition of an isUnmounted property on contexts.

Example:

// Old behavior: this component would continuously resume
// New behavior: pauses at first yield until rerendered
async function *Timer() {
  let i = 0;
  while (true) {
    await new Promise((r) => setTimeout(r, 1000));
    yield <div>{i}</div>
  }
}

Action Items

• Update async generator component logic to pause at first yield when not in a loop.
• Document migration steps and highlight affected patterns in release notes.