The code path changed by this commit isn't only taken when running using Vite. If the site is configured with a base url which is different from `/` but does **not** end with `/` (for example, because `trailingSlash` is set to `never`), the `- 1` results in an off-by-one error when truncating the URL.
By checking if the base url ends with `/`, we can determine the right length for the prefix to truncate.
