#Scroll Behavior

When navigating between routes, @esmx/router automatically manages scroll position to match user expectations. Pushing to a new page scrolls to the top; going back restores the previous scroll position. This mirrors how traditional multi-page websites behave.

#Default Behavior

The router handles scrolling differently based on the navigation type:

• push: Scrolls to top (0, 0)
• replace: Scrolls to top (0, 0)
• back: Restores saved scroll position
• forward: Restores saved scroll position
• go(n): Restores saved scroll position
• pushWindow: Handled by browser
• replaceWindow: Handled by browser

await router.push('/new-page');

await router.back();

This works out of the box with no configuration needed.

#How Scroll Positions Are Saved

When leaving a page (via push, replace, or history navigation), the router saves the current scroll position using two mechanisms:

1. In-memory map: A Map<string, ScrollPosition> keyed by the page's full URL
2. History state: The position is also stored in history.state under the __scroll_position_key property

const scrollPosition = { left: window.scrollX, top: window.scrollY };

scrollPositions.set(currentUrl, scrollPosition);

history.replaceState({
  ...history.state,
  __scroll_position_key: scrollPosition
}, '');

Storing in history.state means scroll positions survive page refreshes — when the user refreshes and then navigates back, the correct scroll position can still be restored.

#Manual Scroll Restoration

The router sets history.scrollRestoration = 'manual' automatically. This tells the browser not to attempt its own scroll restoration, leaving full control to the router.

This is configured during the confirm phase of navigation — you don't need to set it yourself.

#Keeping Scroll Position

Sometimes you don't want navigation to scroll to the top. For example, when switching tabs or filtering content, the user expects to stay where they are:

await router.push({
  path: '/dashboard',
  query: { tab: 'settings' },
  keepScrollPosition: true
});

When keepScrollPosition is set to true:

• The page does not scroll to top
• The current scroll position is not saved (since we're staying at the same position)
• The __keepScrollPosition flag is stored in history.state

This flag is also checked during back/forward navigation — if the target history entry was created with keepScrollPosition: true, scroll restoration is skipped.

#Scroll to Element

The scroll system supports scrolling to a specific element on the page using a CSS selector. You can specify the target via the el property:

• CSS selector strings (e.g., '#my-id', '.my-class', '[data-section]')
• DOM Element references

router.afterEach((to) => {
  if (to.hash) {
    setTimeout(() => {
      const el = document.querySelector(to.hash);
      if (el) {
        el.scrollIntoView({ behavior: 'smooth' });
      }
    }, 100); // wait for DOM to update
  }
});

#Layers and Scroll

Routes opened via layer routing (using pushLayer or createLayer) skip scroll handling entirely. Since layers render in an overlay on top of the current page, scrolling the background page would be disruptive:

await router.pushLayer('/confirm-dialog');

This behavior is built into the router's confirm phase — scroll logic is skipped when router.isLayer is true.

#Scroll Position Flow

Here's the complete flow of how scroll is handled during different navigation types:

#push / replace

1. Save current scroll position for the current URL
2. Perform navigation (update history, mount component)
3. Scroll to (0, 0) — unless keepScrollPosition is true

#back / forward / go

1. Save current scroll position for the current URL
2. Perform navigation (history popstate fires)
3. Wait for DOM update (nextTick)
4. Check if history.state has __keepScrollPosition flag
   → If yes: skip scroll restoration
   → If no: restore saved scroll position for the new URL
     → Falls back to (0, 0) if no saved position exists

#Window Navigation (pushWindow / replaceWindow)

1. Full browser navigation — scroll handled by browser natively

#Summary

• Scroll to top on push/replace: Enabled by default. Pass keepScrollPosition: true to disable.
• Restore scroll on back/forward: Enabled by default. Uses saved positions automatically.
• Browser scroll restoration: Disabled ('manual'). Set automatically by router.
• Layer scroll handling: Skipped. Automatic for layers.
• Persist across page refresh: Via history.state. Automatic.