Implement window.history History API #159

Description#

Implement the History API (window.history) per the WHATWG HTML spec. This enables single-page applications to manage navigation state without full page reloads.

Requirements#

Properties#

• history.length — number of entries in the session history
• history.state — the state object for the current entry (or null)
• history.scrollRestoration — auto or manual

Methods#

• history.pushState(state, title, url) — push a new entry without page reload. URL must be same-origin. State is structured-cloned.
• history.replaceState(state, title, url) — replace the current entry without page reload. Same constraints as pushState.
• history.back() — equivalent to history.go(-1)
• history.forward() — equivalent to history.go(1)
• history.go(delta) — traverse by delta steps. history.go(0) reloads.

Events#

• popstate event fires on the window when the active history entry changes due to traversal (back/forward/go), NOT when pushState/replaceState is called
• Event has a state property containing the state object of the new entry
• popstate fires after the entry is activated

Constraints#

• pushState/replaceState URL must be same-origin as current document
• State object must be serializable (structured clone)
• Rapid pushState calls should be rate-limited to prevent abuse

Acceptance Criteria#

• pushState adds entry and updates URL bar without page reload
• replaceState updates current entry without adding new one
• back()/forward()/go() traverse the history
• popstate event fires with correct state on traversal
• history.length reflects actual entry count
• Cross-origin URL in pushState throws SecurityError
• Unit tests for all History API methods and popstate event