Calculate sidebar expansion for groups with paths on page open

[echojoshchen]

• I confirm that this is an issue rather than a question.

Sidebar groups with paths do not expand the sidebar when directly navigating to these pages.

Bug report

Steps to reproduce

Demo repo: https://github.com/echojoshchen/vuepress-subheading-demo

1. Navigate to /test/sections, the first sidebar group expands instead of the active one.
2. Navigate to /test/section-2, the first sidebar group expands, and no active header is shown.

What is expected?

The sidebar should be expanded to the same layout as when navigating by clicking.

What is actually happening?

The first sidebar group expands instead of the active one.

Other relevant information