Ng-zorro-antd: 官网文档中 modal 自定义位置/垂直居中 demo 存在问题

[lsbbd]

Reproduction link

https://github.com/NG-ZORRO/ng-zorro-antd/blob/65944147b22767bfb3c3c7aec71c16d8ffaf6e76/components/modal/demo/position.ts#L41

Steps to reproduce

1. 打开链接 http://localhost:62754/components/modal/zh#components-modal-demo-position
2. 点击 Vertically centered modal dialog 按钮
3. 缩小窗口宽度至 modal 溢出
4. 尝试滚动到 modal 顶部

What is expected?

正常显示完整的 modal

What is actually happening?

modal 顶部溢出部分被裁切，无法显示

| Environment | Info |
|---|---|
| ng-zorro-antd | 11.0.1 |
| Browser | chrome 87.0.4280.141 |

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可参考 (https://stackoverflow.com/questions/33454533/cant-scroll-to-top-of-flex-item-that-is-overflowing-container)

[hsuanxyz]

@lsbbd Hi, I see that the React version added a centered property for Modal, and the implementation uses an other solution, would you like to submit this contribution for us? This means that instead of accepting #6328, we will be adding a new feature(nzCentered)

[zorro-bot[bot]]

Hello @lsbbd. We totally like your proposal/feedback, welcome to send us a Pull Request for it. Please fill the Pull Request Template here, provide documentation/test cases if needed and make sure CI passed, we will review it soon. Appreciate it advance and we are looking forward to your contribution!

你好 @lsbbd, 我们完全同意你的提议/反馈，欢迎直接在此仓库 创建一个 Pull Request 来解决这个问题。请务必填写 Pull Request 内的预设模板，提供改动所需相应的测试用例、文档等，并确保 CI 通过，我们会尽快进行 Review，提前感谢和期待您的贡献！

[lsbbd]

@lsbbd Hi, I see that the React version added a centered property for Modal, and the implementation uses an other solution, would you like to submit this contribution for us? This means that instead of accepting #6328, we will be adding a new feature(nzCentered)

Okay, I will implement nzCentered