diff --git a/.github/workflows/pr.yaml b/.github/workflows/pr.yaml
index cd68a1d70..96a4667d1 100644
--- a/.github/workflows/pr.yaml
+++ b/.github/workflows/pr.yaml
@@ -25,11 +25,13 @@ jobs:
           # and should no longer be modified in app/.
           # Calculator components (report builder, household, charts, etc.) are
           # still legitimately modified in app/ since calculator-app/ imports from there.
+          # Note: HomeHeader.tsx and homeHeader/ were ported to website but
+          # are still actively used by the calculator-app (StandardLayout →
+          # HomeHeader). They are now de facto calculator components, so
+          # they're intentionally NOT in this guard list.
           WEBSITE_BLOCKED=$(echo "$ALL_CHANGED" | grep -E \
             -e '^app/src/components/home/' \
             -e '^app/src/components/shared/static/' \
-            -e '^app/src/components/shared/HomeHeader\.tsx' \
-            -e '^app/src/components/homeHeader/' \
             -e '^app/src/components/Footer\.tsx' \
             -e '^app/src/components/FooterSubscribe\.tsx' \
             -e '^app/src/components/blog/BlogPostCard\.tsx' \
diff --git a/CLAUDE.md b/CLAUDE.md
index 719a38e56..a766ad8b6 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -5,7 +5,8 @@
 The `app/` directory contains the legacy Vite build. Some parts have been ported to Next.js, others are still active.
 
 **Ported to website/ (do NOT modify in app/):**
-- Website components: `home/`, `shared/static/`, `homeHeader/`, `Footer.tsx`, `FooterSubscribe.tsx`, `blog/BlogPostCard.tsx`, `blog/BlogPostGrid.tsx`, `blog/ResearchFilters.tsx`
+- Website components: `home/`, `shared/static/`, `Footer.tsx`, `FooterSubscribe.tsx`, `blog/BlogPostCard.tsx`, `blog/BlogPostGrid.tsx`, `blog/ResearchFilters.tsx`
+  - Note: `homeHeader/` and `shared/HomeHeader.tsx` were also ported to `website/src/components/Header.tsx`, but the calculator-app still renders `HomeHeader` via `StandardLayout`, so these remain editable in `app/`. Keep the calculator header in sync with the website header when the website one changes.
 - Website pages: `Home`, `Research`, `Blog`, `Team`, `Supporters`, `Donate`, `Privacy`, `Terms`, `Brand*`, `Citations`, `AppPage`
 - `vercel.json` (root) — new rewrites go in `website/next.config.ts`
 
diff --git a/app/src/components/homeHeader/MobileMenu.tsx b/app/src/components/homeHeader/MobileMenu.tsx
index e23cc37d4..9d62652a3 100644
--- a/app/src/components/homeHeader/MobileMenu.tsx
+++ b/app/src/components/homeHeader/MobileMenu.tsx
@@ -58,7 +58,7 @@ export default function MobileMenu({ opened, onOpen, onClose, navItems }: Mobile
                     className="tw:flex tw:flex-col"
                     style={{ gap: spacing.xs, paddingLeft: spacing.md }}
                   >
-                    {item.dropdownItems.map((dropdownItem) => (
+                    {item.dropdownItems.flatMap((dropdownItem) => [
                       <a
                         key={dropdownItem.label}
                         href={dropdownItem.href}
@@ -72,8 +72,26 @@ export default function MobileMenu({ opened, onOpen, onClose, navItems }: Mobile
                         }}
                       >
                         {dropdownItem.label}
-                      </a>
-                    ))}
+                      </a>,
+                      ...(dropdownItem.children ?? []).map((grandchild) => (
+                        <a
+                          key={`${dropdownItem.label}-${grandchild.label}`}
+                          href={grandchild.href}
+                          onClick={grandchild.href ? undefined : grandchild.onClick}
+                          style={{
+                            color: colors.text.inverse,
+                            textDecoration: 'none',
+                            fontWeight: typography.fontWeight.normal,
+                            fontSize: typography.fontSize.sm,
+                            fontFamily: typography.fontFamily.primary,
+                            paddingLeft: spacing.md,
+                            opacity: 0.85,
+                          }}
+                        >
+                          {grandchild.label}
+                        </a>
+                      )),
+                    ])}
                   </div>
                 </div>
               ) : (
diff --git a/app/src/components/homeHeader/NavItem.tsx b/app/src/components/homeHeader/NavItem.tsx
index 2dd4ecc01..d1b2d1f84 100644
--- a/app/src/components/homeHeader/NavItem.tsx
+++ b/app/src/components/homeHeader/NavItem.tsx
@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useRef, useState } from 'react';
 import { IconChevronDown } from '@tabler/icons-react';
 import { AppLink } from '@/components/AppLink';
+import { useAppPathname } from '@/contexts/LocationContext';
 import { useAppNavigate } from '@/contexts/NavigationContext';
 import { colors, typography } from '@/designTokens';
 
@@ -8,6 +9,8 @@ export interface DropdownItem {
   label: string;
   onClick?: () => void;
   href?: string;
+  /** Nested children rendered indented under this item. One level deep only. */
+  children?: DropdownItem[];
 }
 
 export interface NavItemSetup {
@@ -22,47 +25,130 @@ interface NavItemProps {
   setup: NavItemSetup;
 }
 
+const NAV_ITEM_PADDING_X = 14;
+const NAV_UNDERLINE_INSET = 10;
+const DROPDOWN_GAP = 10;
+const HOVER_OPEN_DELAY_MS = 100;
+const HOVER_CLOSE_DELAY_MS = 200;
+
 const navItemStyle: React.CSSProperties = {
   color: colors.text.inverse,
   fontWeight: typography.fontWeight.medium,
   fontSize: '15px',
   fontFamily: typography.fontFamily.primary,
   textDecoration: 'none',
-  padding: '6px 14px',
-  borderRadius: '6px',
-  transition: 'background-color 0.15s ease',
+  padding: `8px ${NAV_ITEM_PADDING_X}px`,
   letterSpacing: '0.01em',
+  position: 'relative',
 };
 
-const hoverHandlers = {
-  onMouseEnter: (e: React.MouseEvent<HTMLElement>) => {
-    e.currentTarget.style.backgroundColor = 'rgba(255, 255, 255, 0.12)';
-  },
-  onMouseLeave: (e: React.MouseEvent<HTMLElement>) => {
-    e.currentTarget.style.backgroundColor = 'transparent';
-  },
-};
+function NavUnderline({ visible }: { visible: boolean }) {
+  return (
+    <span
+      aria-hidden="true"
+      style={{
+        position: 'absolute',
+        left: `${NAV_UNDERLINE_INSET}px`,
+        right: `${NAV_UNDERLINE_INSET}px`,
+        bottom: '2px',
+        height: '2px',
+        borderRadius: '2px',
+        backgroundColor: colors.text.inverse,
+        transform: visible ? 'scaleX(1)' : 'scaleX(0)',
+        transformOrigin: 'center',
+        transition: 'transform 0.25s cubic-bezier(0.4, 0, 0.2, 1)',
+        pointerEvents: 'none',
+      }}
+    />
+  );
+}
 
-/**
- * Check if href is a relative path (internal) vs absolute URL (external).
- */
 function isInternalHref(href: string | undefined): href is string {
   return !!href && href.startsWith('/');
 }
 
-/**
- * Apple-style dropdown panel with smooth height reveal and content fade.
- */
-function AppleDropdown({
+function itemMatches(href: string | undefined, pathname: string): boolean {
+  if (!href || !isInternalHref(href)) {
+    return false;
+  }
+  return pathname === href || pathname.startsWith(`${href}/`);
+}
+
+function isItemActive(item: NavItemSetup, pathname: string): boolean {
+  if (item.hasDropdown && item.dropdownItems) {
+    return item.dropdownItems.some(
+      (child) =>
+        itemMatches(child.href, pathname) ||
+        (child.children?.some((grand) => itemMatches(grand.href, pathname)) ?? false)
+    );
+  }
+  return itemMatches(item.href, pathname);
+}
+
+function DropdownRow({
+  item,
+  depth,
+  index,
+  visible,
+  onSelect,
+}: {
+  item: DropdownItem;
+  depth: number;
+  index: number;
+  visible: boolean;
+  onSelect: (item: DropdownItem) => void;
+}) {
+  const isChild = depth > 0;
+  return (
+    <button
+      type="button"
+      onClick={() => onSelect(item)}
+      style={{
+        display: 'flex',
+        alignItems: 'center',
+        width: '100%',
+        textAlign: 'left',
+        padding: `${isChild ? 8 : 11}px 16px ${isChild ? 8 : 11}px ${16 + depth * 16}px`,
+        borderRadius: '10px',
+        border: 'none',
+        background: 'transparent',
+        cursor: 'pointer',
+        fontSize: isChild ? '13px' : '14px',
+        fontFamily: typography.fontFamily.primary,
+        fontWeight: isChild ? typography.fontWeight.medium : typography.fontWeight.semibold,
+        color: isChild ? colors.primary[700] : colors.primary[800],
+        transition: `background-color 0.12s ease 0ms, color 0.12s ease 0ms, opacity 0.3s ease ${
+          visible ? index * 30 : 0
+        }ms`,
+        opacity: visible ? 1 : 0,
+        lineHeight: '1.3',
+        letterSpacing: '-0.01em',
+      }}
+      onMouseEnter={(e) => {
+        e.currentTarget.style.backgroundColor = colors.primary[500];
+        e.currentTarget.style.color = colors.text.inverse;
+      }}
+      onMouseLeave={(e) => {
+        e.currentTarget.style.backgroundColor = 'transparent';
+        e.currentTarget.style.color = isChild ? colors.primary[700] : colors.primary[800];
+      }}
+    >
+      {item.label}
+    </button>
+  );
+}
+
+function DropdownPanel({
   items,
   open,
   onClose,
+  onSelect,
 }: {
   items: DropdownItem[];
   open: boolean;
   onClose: () => void;
+  onSelect: (item: DropdownItem) => void;
 }) {
-  const nav = useAppNavigate();
   const contentRef = useRef<HTMLDivElement>(null);
   const [contentHeight, setContentHeight] = useState(0);
   const [visible, setVisible] = useState(false);
@@ -78,49 +164,39 @@ function AppleDropdown({
     }
   }, [open]);
 
-  const handleSelect = useCallback(
-    (item: DropdownItem) => {
-      onClose();
-      if (item.href) {
-        if (isInternalHref(item.href)) {
-          nav.push(item.href);
-        } else {
-          window.location.href = item.href;
-        }
-      } else if (item.onClick) {
-        item.onClick();
-      }
-    },
-    [nav, onClose]
-  );
-
   if (!open && contentHeight === 0) {
     return null;
   }
 
+  const rows: Array<{ item: DropdownItem; depth: number }> = [];
+  for (const item of items) {
+    rows.push({ item, depth: 0 });
+    if (item.children) {
+      for (const child of item.children) {
+        rows.push({ item: child, depth: 1 });
+      }
+    }
+  }
+
+  const handleSelect = (item: DropdownItem) => {
+    onClose();
+    onSelect(item);
+  };
+
   return (
-    <>
-      {/* Invisible click-away layer (no dim) — sits below the header's z-index */}
-      {/* eslint-disable-next-line jsx-a11y/click-events-have-key-events, jsx-a11y/no-static-element-interactions */}
+    <div
+      style={{
+        position: 'absolute',
+        top: '100%',
+        left: '50%',
+        transform: 'translateX(-50%)',
+        paddingTop: `${DROPDOWN_GAP}px`,
+        zIndex: 1001,
+      }}
+    >
       <div
-        onClick={onClose}
         style={{
-          position: 'fixed',
-          inset: 0,
-          zIndex: 999,
-          cursor: 'default',
-        }}
-      />
-      {/* Dropdown panel */}
-      <div
-        style={{
-          position: 'absolute',
-          top: '100%',
-          left: '50%',
-          transform: visible
-            ? 'translateX(-50%) translateY(0)'
-            : 'translateX(-50%) translateY(-8px)',
-          marginTop: '10px',
+          transform: visible ? 'translateY(0)' : 'translateY(-8px)',
           minWidth: '220px',
           overflow: 'hidden',
           maxHeight: visible ? `${contentHeight}px` : '0px',
@@ -128,68 +204,55 @@ function AppleDropdown({
           transition:
             'max-height 0.3s cubic-bezier(0.4, 0, 0.2, 1), opacity 0.2s ease, transform 0.3s cubic-bezier(0.4, 0, 0.2, 1)',
           borderRadius: '14px',
-          background: `linear-gradient(135deg, rgba(255,255,255,0.97), rgba(240,249,255,0.95))`,
+          background: 'linear-gradient(135deg, rgba(255,255,255,0.97), rgba(240,249,255,0.95))',
           backdropFilter: 'blur(24px) saturate(200%)',
           WebkitBackdropFilter: 'blur(24px) saturate(200%)',
           boxShadow:
             '0 20px 60px rgba(0, 0, 0, 0.15), 0 4px 16px rgba(0, 0, 0, 0.06), inset 0 0 0 1px rgba(255, 255, 255, 0.6)',
-          zIndex: 1001,
         }}
       >
         <div ref={contentRef} style={{ padding: '8px' }}>
-          {items.map((item, i) => (
-            <button
-              key={item.label}
-              type="button"
-              onClick={() => handleSelect(item)}
-              style={{
-                display: 'flex',
-                alignItems: 'center',
-                width: '100%',
-                textAlign: 'left',
-                padding: '11px 16px',
-                borderRadius: '10px',
-                border: 'none',
-                background: 'transparent',
-                cursor: 'pointer',
-                fontSize: '14px',
-                fontFamily: typography.fontFamily.primary,
-                fontWeight: typography.fontWeight.semibold,
-                color: colors.primary[800],
-                transition: 'background-color 0.12s ease, color 0.12s ease, opacity 0.3s ease',
-                transitionDelay: visible ? `${i * 50}ms` : '0ms',
-                opacity: visible ? 1 : 0,
-                lineHeight: '1.3',
-                letterSpacing: '-0.01em',
-              }}
-              onMouseEnter={(e) => {
-                e.currentTarget.style.backgroundColor = colors.primary[500];
-                e.currentTarget.style.color = colors.text.inverse;
-              }}
-              onMouseLeave={(e) => {
-                e.currentTarget.style.backgroundColor = 'transparent';
-                e.currentTarget.style.color = colors.primary[800];
-              }}
-            >
-              {item.label}
-            </button>
+          {rows.map(({ item, depth }, i) => (
+            <DropdownRow
+              key={`${item.label}-${item.href ?? ''}-${depth}`}
+              item={item}
+              depth={depth}
+              index={i}
+              visible={visible}
+              onSelect={handleSelect}
+            />
           ))}
         </div>
       </div>
-    </>
+    </div>
   );
 }
 
-/**
- * Reusable navigation item component.
- * Can be either a simple link or a dropdown menu.
- */
 export default function NavItem({ setup }: NavItemProps) {
   const { label, onClick, href, hasDropdown, dropdownItems } = setup;
+  const nav = useAppNavigate();
+  const pathname = useAppPathname();
   const [dropdownOpen, setDropdownOpen] = useState(false);
+  const [hovered, setHovered] = useState(false);
   const containerRef = useRef<HTMLDivElement>(null);
+  const openTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
+  const closeTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
+
+  const active = isItemActive(setup, pathname);
+
+  const clearTimers = useCallback(() => {
+    if (openTimerRef.current) {
+      clearTimeout(openTimerRef.current);
+      openTimerRef.current = null;
+    }
+    if (closeTimerRef.current) {
+      clearTimeout(closeTimerRef.current);
+      closeTimerRef.current = null;
+    }
+  }, []);
+
+  useEffect(() => clearTimers, [clearTimers]);
 
-  // Close on click outside
   useEffect(() => {
     if (!dropdownOpen) {
       return;
@@ -199,33 +262,69 @@ export default function NavItem({ setup }: NavItemProps) {
         setDropdownOpen(false);
       }
     }
-    document.addEventListener('mousedown', handleClick);
-    return () => document.removeEventListener('mousedown', handleClick);
-  }, [dropdownOpen]);
-
-  // Close on Escape
-  useEffect(() => {
-    if (!dropdownOpen) {
-      return;
-    }
     function handleKey(e: KeyboardEvent) {
       if (e.key === 'Escape') {
         setDropdownOpen(false);
       }
     }
+    document.addEventListener('mousedown', handleClick);
     document.addEventListener('keydown', handleKey);
-    return () => document.removeEventListener('keydown', handleKey);
+    return () => {
+      document.removeEventListener('mousedown', handleClick);
+      document.removeEventListener('keydown', handleKey);
+    };
   }, [dropdownOpen]);
 
+  const handleMouseEnter = () => {
+    setHovered(true);
+    if (!hasDropdown) {
+      return;
+    }
+    clearTimers();
+    openTimerRef.current = setTimeout(() => setDropdownOpen(true), HOVER_OPEN_DELAY_MS);
+  };
+
+  const handleMouseLeave = () => {
+    setHovered(false);
+    if (!hasDropdown) {
+      return;
+    }
+    clearTimers();
+    closeTimerRef.current = setTimeout(() => setDropdownOpen(false), HOVER_CLOSE_DELAY_MS);
+  };
+
+  const underlineVisible = active || hovered || dropdownOpen;
+
+  const handleDropdownSelect = (item: DropdownItem) => {
+    if (item.href) {
+      if (isInternalHref(item.href)) {
+        nav.push(item.href);
+      } else {
+        window.location.href = item.href;
+      }
+    } else if (item.onClick) {
+      item.onClick();
+    }
+  };
+
   if (hasDropdown && dropdownItems) {
     return (
-      <div ref={containerRef} style={{ position: 'relative' }}>
+      <div
+        ref={containerRef}
+        style={{ position: 'relative' }}
+        onMouseEnter={handleMouseEnter}
+        onMouseLeave={handleMouseLeave}
+      >
         <button
           type="button"
           onClick={() => {
             onClick?.();
             setDropdownOpen((prev) => !prev);
           }}
+          onFocus={() => setHovered(true)}
+          onBlur={() => setHovered(false)}
+          aria-expanded={dropdownOpen}
+          aria-haspopup="true"
           style={{
             ...navItemStyle,
             background: 'transparent',
@@ -235,7 +334,6 @@ export default function NavItem({ setup }: NavItemProps) {
             alignItems: 'center',
             gap: '4px',
           }}
-          {...hoverHandlers}
         >
           <span>{label}</span>
           <IconChevronDown
@@ -247,29 +345,47 @@ export default function NavItem({ setup }: NavItemProps) {
               transform: dropdownOpen ? 'rotate(180deg)' : 'rotate(0deg)',
             }}
           />
+          <NavUnderline visible={underlineVisible} />
         </button>
-        <AppleDropdown
+        <DropdownPanel
           items={dropdownItems}
           open={dropdownOpen}
           onClose={() => setDropdownOpen(false)}
+          onSelect={handleDropdownSelect}
         />
       </div>
     );
   }
 
-  // Relative paths use AppLink for SPA behavior
   if (isInternalHref(href)) {
     return (
-      <AppLink to={href} style={navItemStyle} {...hoverHandlers}>
+      <AppLink
+        to={href}
+        style={navItemStyle}
+        onMouseEnter={handleMouseEnter}
+        onMouseLeave={handleMouseLeave}
+        onFocus={() => setHovered(true)}
+        onBlur={() => setHovered(false)}
+        aria-current={active ? 'page' : undefined}
+      >
         {label}
+        <NavUnderline visible={underlineVisible} />
       </AppLink>
     );
   }
 
-  // Absolute URLs use standard anchor tag
   return (
-    <a href={href} onClick={href ? undefined : onClick} style={navItemStyle} {...hoverHandlers}>
+    <a
+      href={href}
+      onClick={href ? undefined : onClick}
+      style={navItemStyle}
+      onMouseEnter={handleMouseEnter}
+      onMouseLeave={handleMouseLeave}
+      onFocus={() => setHovered(true)}
+      onBlur={() => setHovered(false)}
+    >
       {label}
+      <NavUnderline visible={underlineVisible} />
     </a>
   );
 }
diff --git a/app/src/components/shared/HomeHeader.tsx b/app/src/components/shared/HomeHeader.tsx
index 04bd839ca..cfc5c57d1 100644
--- a/app/src/components/shared/HomeHeader.tsx
+++ b/app/src/components/shared/HomeHeader.tsx
@@ -14,6 +14,11 @@ export default function HeaderNavigation({ navbarOpened, onToggleNavbar }: Heade
   const [opened, { open, close }] = useDisclosure(false);
   const { getWebsitePath, countryId } = useWebsitePath();
 
+  // The model explorer, API docs, and Python docs are separate apps served via
+  // Vercel rewrites — use absolute URLs so the browser performs a full
+  // navigation across zones rather than routing client-side.
+  const modelBase = `${WEBSITE_URL}/${countryId}/model`;
+
   const navItems: NavItemSetup[] = [
     {
       label: 'Research',
@@ -22,18 +27,42 @@ export default function HeaderNavigation({ navbarOpened, onToggleNavbar }: Heade
     },
     {
       label: 'Model',
-      // Always use an absolute URL — the model explorer is a separate app served
-      // via Vercel rewrite, so React Router must not intercept this link.
-      href: `${WEBSITE_URL}/${countryId}/model`,
-      hasDropdown: false,
+      hasDropdown: true,
+      dropdownItems: [
+        {
+          label: 'Rules',
+          href: `${modelBase}/rules`,
+          children: [
+            { label: 'Coverage', href: `${modelBase}/rules/coverage` },
+            { label: 'Parameters', href: `${modelBase}/rules/parameters` },
+            { label: 'Variables', href: `${modelBase}/rules/variables` },
+          ],
+        },
+        {
+          label: 'Data',
+          href: `${modelBase}/data`,
+          children: [
+            { label: 'Pipeline', href: `${modelBase}/data/pipeline` },
+            { label: 'Calibration', href: `${modelBase}/data/calibration` },
+            { label: 'Validation', href: `${modelBase}/data/validation` },
+          ],
+        },
+        {
+          label: 'Behavioral responses',
+          href: `${modelBase}/behavioral`,
+        },
+      ],
     },
     {
       label: 'API',
-      // Always use an absolute URL — the API docs are a separate app served
-      // via Vercel rewrite, so React Router must not intercept this link.
       href: `${WEBSITE_URL}/${countryId}/api`,
       hasDropdown: false,
     },
+    {
+      label: 'Python',
+      href: `${WEBSITE_URL}/${countryId}/python`,
+      hasDropdown: false,
+    },
     {
       label: 'About',
       hasDropdown: true,
@@ -41,6 +70,7 @@ export default function HeaderNavigation({ navbarOpened, onToggleNavbar }: Heade
         { label: 'Team', href: getWebsitePath('/team') },
         { label: 'Supporters', href: getWebsitePath('/supporters') },
         { label: 'Citations', href: getWebsitePath('/citations') },
+        { label: 'Events', href: getWebsitePath('/events') },
       ],
     },
     {
diff --git a/changelog_entry.yaml b/changelog_entry.yaml
index ae7eaebae..05a557f39 100644
--- a/changelog_entry.yaml
+++ b/changelog_entry.yaml
@@ -5,3 +5,4 @@
       - Add UK fuel duty rise cancellation analysis dashboard at /uk/cancelling-fuel-duty-rise
     changed:
       - Proxy UK Python package docs at /uk/python and UK API docs at /uk/api so the country-parameterized "Python" and "API" nav links work on UK pages
+      - Align the calculator app header (e.g. /us/reports) with the main website header — hover-open dropdowns, per-item underline, Model dropdown with sub-items, Python link, and Events under About