Views custom URLs prefixed with / don't work in subdirectories

Problem/Motivation

Views displays support custom URLs, and when those displays have exposed forms, the form's action will be the display's custom URL. So far, so good.

Unfortunately, this breaks if Drupal is running in a subdirectory. For example, let's say you have a view whose custom URL is /search, and Drupal is running at https://example.com/foo. The exposed form's action will be /search, which will be a 404.

Proposed resolution

The exposed form needs to actually use URL processing, probably converting /whatever to base:/whatever. Since CI runs Drupal in a subdirectory, this points to a clear gap in test coverage.

User interface changes

None.

Introduced terminology

None.

API changes

None.

Data model changes

None.

Release notes snippet

None needed, it's a bug fix.