How do you reliably interact with elements inside an iframe when the iframe content is loaded asynchronously and might not be immediately visible?

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### Question
How do you reliably interact with elements inside an iframe when the iframe content is loaded asynchronously and might not be immediately visible?

### Answer
When testing a single-page application (SPA) that embeds complex third-party widgets within an `<iframe>`, you encounter a race condition: the main page loads quickly, but the content within the iframe loads asynchronously after a delayed API call.

You need to locate and interact with a specific button inside that iframe, but simply calling `frame.locator('button#submit')` immediately after switching into the frame fails intermittently because the button element hasn't been rendered yet.

What is the most robust, production-grade pattern in Playwright to ensure the interaction only occurs *after* the element is fully present and actionable inside the iframe, without relying on arbitrary `page.waitForTimeout(1000)`? Provide the specific Playwright API calls necessary to achieve this reliability.

How do you reliably interact with elements inside an iframe when the iframe content is loaded asynchronously and might not be immediately visible?

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