Cannot operate when embedding iframes
When embedding a Pyxel game in an iframe, if you focus on another element and then focus again on the game in the iframe, the game will become inoperable.
However, the game can be operated only when the bottom part of the iframe is focused.
![2023-02-20_20h28_28](https://user-images.githubusercontent.com/93308122/220097713-e63a8ec2-7cea-4ba9-9e29-a9e70f8fe54d.png)

## Reproduction procedure
1. access the HTML code below to start the game in the iframe.
2. When the game loads, focus on the `<p>` element.
3. When you focus on the game in the iframe again, the game is not operable.

## Code that caused the problem
This is the code that embeds 30SecondsOfDaylight in Examples.
This code using Bootstrap.
```html
<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.2/dist/css/bootstrap.min.css">
        <title>Pyxel Test</title>
    </head>
    <body>
        <div class="container mt-4 mx-5 px-5 w-75 mx-auto">
                    <div class="ratio ratio-4x3" id="game">
                        <iframe class="rounded" src="https://kitao.github.io/pyxel/wasm/examples/30SecondsOfDaylight.html" frameborder="0"></iframe>
                    </div>
                <p>TEXT TEXT TEXT TEXT</p>
        </div>
    </body>
</html>
```

## Execution environment
Windows 11, Firefox 109.0.1