Local navigation dropdown misplaced without sidebar

[harsssh]

Describe the bug

When a site has no sidebar, the local navigation dropdown is incorrectly positioned. It appears to be offset to the right by the sidebar width, even though there is no sidebar present.

https://github.com/vuejs/vitepress/blob/d200f4e12543016e30c51661fb46d4185ed6be95/src/client/theme-default/components/VPLocalNavOutlineDropdown.vue#L161-L167

Reproduction

https://stackblitz.com/edit/vite-dxfigx?file=docs%2F.vitepress%2Fconfig.ts

Expected behavior

When there is no sidebar, the dropdown should still appear directly below the "On this page" text, aligned as shown in the image.

System Info

System:
  OS: macOS 14.2.1
  CPU: (8) arm64 Apple M2
  Memory: 140.70 MB / 8.00 GB
  Shell: 5.9 - /bin/zsh
Binaries:
  Node: 22.11.0 - ~/.asdf/installs/nodejs/22.11.0/bin/node
  Yarn: 1.22.22 - ~/.asdf/installs/nodejs/22.11.0/bin/yarn
  npm: 10.9.0 - ~/.asdf/plugins/nodejs/shims/npm
  bun: 1.1.33 - ~/.bun/bin/bun
Browsers:
  Chrome: 130.0.6723.117
  Safari: 17.2.1
npmPackages:
  vitepress: ^1.5.0 => 1.5.0

Additional context

No response

Validations

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

[brc-dd]

--vp-sidebar-width should be 0 when there is no sidebar

[harsssh]

I'd like to work on fixing this bug. May I take this on?

[brc-dd]

Yeah sure! Thanks.