Drawer is hide below to WebviewScaffold.

## System info

Issue occurs on: iOS / Android / both
Plugin version: xxx
Flutter doctor output:

```
paste it here...
```

## Steps to Reproduce

WebviewScaffold is covered the whole drawer menu.

1. ...
2. ...
3. ...

## Logs

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首先在 iOS 和 Android 上复现报告的 WebviewScaffold 与 drawer 交互，并补充缺失的复现和系统细节。参考 Flutter WebView 和 drawer 文档确认预期的分层行为；完成标准是 drawer 菜单可见，而不是被 WebviewScaffold 覆盖。

由索引模型根据 Issue 内容生成。