fix(docs-infra): correctly handle in-page anchor links (#43460)

Previously, anchor links that were supposed to point to fragments inside
the current page (e.g. `href="#something"`) would end up navigating to
the homepage (`/`). This was due to them being resolved relative to the
base URL, which is set to `/` (via `<base href="/">`). See also
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base#in-page_anchors.

This commit fixes this issue by handling such links specially in
`LocationService#handleAnchorClick()` and prepending the current path to
the URL.

This will also be useful for implementing a "skip to content" link as
described in #42936.

PR Close #43460

aio/src/app/shared/location.service.spec.ts

@@ -436,6 +436,14 @@ describe('LocationService', () => {
         expect(result).toBe(false);
       });
 
+      it('local fragment-only URL', () => {
+        location.internalPath = '/some/path';
+        anchor.href = '#some-fragment';
+        const result = service.handleAnchorClick(anchor);
+        expect(service.go).toHaveBeenCalledWith('/some/path#some-fragment');
+        expect(result).toBe(false);
+      });
+
       it('local url with query params', () => {
         anchor.href = 'some/local/url?query=xxx&other=yyy';
         const result = service.handleAnchorClick(anchor);

aio/src/app/shared/location.service.ts

@@ -145,11 +145,16 @@ export class LocationService {
     }
 
     const { pathname, search, hash } = anchor;
-    const relativeUrl = pathname + search + hash;
+    // Fix in-page anchors that are supposed to point to fragments inside the page, but are resolved
+    // relative the the root path (`/`), due to the base URL being set to `/`.
+    // (See https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base#in-page_anchors.)
+    const isInPageAnchor = anchor.getAttribute('href')?.startsWith('#') ?? false;
+    const correctPathname = isInPageAnchor ? this.location.path() : pathname;
+    const relativeUrl = correctPathname + search + hash;
     this.urlParser.href = relativeUrl;
 
     // don't navigate if external link or has extension
-    if ( anchor.href !== this.urlParser.href ||
+    if ( (!isInPageAnchor && anchor.href !== this.urlParser.href) ||
          !/\/[^/.]*$/.test(pathname) ) {
       return true;
     }