Implement fragment navigation #161

Description#

Implement same-document fragment navigation: clicking a link with href="#section" should scroll to the element with id="section" without reloading the page.

Requirements#

Fragment Navigation#

• Clicking <a href="#id"> scrolls to the element with matching id attribute
• Clicking <a href="#"> scrolls to the top of the document
• Clicking <a href=""> with no fragment reloads (not fragment nav)
• If target id is not found, try matching <a name="..."> for legacy compatibility
• If no matching element is found, do nothing (don't scroll)
• Smooth scroll support: respect scroll-behavior: smooth CSS property (stretch goal)

URL Update#

• Update the URL fragment in the address bar
• Update location.hash
• Push a new history entry for the fragment change

Events#

• Fire hashchange event on the window with oldURL and newURL properties
• hashchange fires AFTER the URL is updated and scroll is performed
• Do NOT fire hashchange for cross-document navigations (only same-document)

Same-Document Detection#

• Navigation is same-document if the only difference is the fragment
• Same scheme, host, port, path, and query — only fragment differs
• pushState navigations are also same-document (handled by History API issue)

Acceptance Criteria#

• Clicking <a href="#id"> scrolls to the matching element
• URL in address bar updates with the fragment
• New history entry is created for fragment navigation
• Back button returns to previous scroll position
• hashchange event fires with correct oldURL and newURL
• No page reload occurs for fragment-only navigations
• Unit tests for fragment matching and hashchange event