'Return to top' button is always visible

[Unbinilium]

Describe the bug

Hello, I noticed that in home layout, on mobile devices (with narrow viewport width), the 'Return to top' button is always visible even you're already at the top, and I have difficulties in hiding this button.

Reproduction

1. Use a phone browser (with narrow viewport width).
2. Open https://vitepress.dev, then you'll see the 'Return to top' button is visible.

Expected behavior

1. It can be automatically hide or show depend on the page location.
2. It can be explicitly disable in configurations, like themeConfig: { ... returnToTop: false, ...}.

System Info

System:
    OS: macOS 13.3.1
    CPU: (8) arm64 Apple M1
    Memory: 106.47 MB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.0.0 - /opt/homebrew/bin/node
    npm: 9.6.4 - /opt/homebrew/bin/npm
  Browsers:
    Chrome: 112.0.5615.137
    Safari: 16.4
  npmPackages:
    vitepress: ^1.0.0-alpha.74 => 1.0.0-alpha.74

Additional context

Screen.Recording.2023-04-29.at.13.51.35.mov

Validations

• Check if you're on the latest VitePress version.
• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.

[ATQQ]

I found out today, too