Slide one shared grape pill between mobile nav items

The mobile bottom bar now draws a single absolutely positioned grape pill
(top 10 px, height 42 px) behind its items and implements handoff Motion
section 1: on a tab change it slides the pill from the old item's left and
width to the new one's over 420 ms on the --ease-spring curve, FLIPs every
moved item with the same timing, and fades the new label in (220 ms
ease-out after 120 ms). Icon colour crossfades via transition color .2s.
Under prefers-reduced-motion the pill jumps with no animation. The pill is
re-placed on resize and once document.fonts.ready resolves. Each item
keeps the full 62 px bar height as its tap target.

Maestro task d340ae00-3c2a-48cf-9ac6-35e0b8ed5a7c

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/shell/BottomBar.tsx

@@ -1,23 +1,127 @@
-import { NavLink } from 'react-router'
+import { useLayoutEffect, useRef } from 'react'
+import { NavLink, useLocation } from 'react-router'
 import { Icon } from '../ui/Icon.tsx'
 import { navItems } from './navItems.ts'
 import styles from './BottomBar.module.css'
 
+// The handoff's Motion §1 timing. SPRING is the --ease-spring token's value.
+const SLIDE_MS = 420
+const SPRING = 'cubic-bezier(.34, 1.4, .64, 1)'
+const LABEL_MS = 220
+const LABEL_DELAY_MS = 120
+const REDUCED_MOTION_QUERY = '(prefers-reduced-motion: reduce)'
+
+// Where the pill and every item sat when the pill was last placed, as layout
+// offsets within the bar (offsets ignore transforms, so a running FLIP does
+// not skew them).
+type Geometry = {
+  active: HTMLElement | null
+  pill: { left: number; width: number }
+  itemLefts: number[]
+}
+
 // Mobile nav: a dark pill bar floating over the bottom of the screen. Inactive
-// items show only their icon; the active item adds its label on a grape pill.
+// items show only their icon; the active item adds its label. One shared
+// grape pill sits behind the items and slides to the active one.
 export function BottomBar() {
+  const barRef = useRef<HTMLElement>(null)
+  const pillRef = useRef<HTMLSpanElement>(null)
+  const placed = useRef<Geometry | null>(null)
+  const { pathname } = useLocation()
+
+  // After each navigation: if the active item changed, move the pill to it and,
+  // motion allowed, slide the pill, FLIP the items and fade the label in.
+  useLayoutEffect(() => {
+    const bar = barRef.current!
+    const pill = pillRef.current!
+    const active = activeItem(bar)
+    const before = placed.current
+    if (before && before.active === active) return
+    placed.current = place(bar, pill, active)
+    if (!before || !active || window.matchMedia(REDUCED_MOTION_QUERY).matches) return
+
+    const after = placed.current
+    if (before.active) {
+      pill.animate(
+        [
+          { left: `${before.pill.left}px`, width: `${before.pill.width}px` },
+          { left: `${after.pill.left}px`, width: `${after.pill.width}px` },
+        ],
+        { duration: SLIDE_MS, easing: SPRING },
+      )
+    }
+    items(bar).forEach((item, i) => {
+      const dx = before.itemLefts[i]! - after.itemLefts[i]!
+      if (dx === 0) return
+      item.animate([{ transform: `translateX(${dx}px)` }, { transform: 'translateX(0)' }], {
+        duration: SLIDE_MS,
+        easing: SPRING,
+      })
+    })
+    active.querySelector(`.${styles.label}`)!.animate(
+      [
+        { opacity: 0, transform: 'translateX(-4px)' },
+        { opacity: 1, transform: 'none' },
+      ],
+      { duration: LABEL_MS, delay: LABEL_DELAY_MS, easing: 'ease-out', fill: 'backwards' },
+    )
+  }, [pathname])
+
+  // Item widths change with the viewport and once the web fonts arrive, so
+  // re-place the pill (without animating) on both. A failed font load
+  // rejects fonts.ready and is left to surface as an unhandled rejection.
+  useLayoutEffect(() => {
+    const bar = barRef.current!
+    const pill = pillRef.current!
+    let mounted = true
+    const replace = () => {
+      if (mounted) placed.current = place(bar, pill, activeItem(bar))
+    }
+    window.addEventListener('resize', replace)
+    // jsdom has no document.fonts; every browser the app targets does.
+    if (document.fonts) void document.fonts.ready.then(replace)
+    return () => {
+      mounted = false
+      window.removeEventListener('resize', replace)
+    }
+  }, [])
+
   return (
-    <nav aria-label="Primary" className={styles.bar}>
+    <nav ref={barRef} aria-label="Primary" className={styles.bar}>
+      <span ref={pillRef} className={styles.pill} aria-hidden="true" />
       {navItems.map((item) => (
         <NavLink key={item.to} to={item.to} end={item.end} className={styles.item}>
           {({ isActive }) => (
-            <span className={isActive ? `${styles.pill} ${styles.active}` : styles.pill}>
+            <>
               <Icon icon={item.icon} size="nav" />
               <span className={isActive ? styles.label : styles.hiddenLabel}>{item.label}</span>
-            </span>
+            </>
           )}
         </NavLink>
       ))}
     </nav>
   )
 }
+
+function items(bar: HTMLElement): HTMLElement[] {
+  return Array.from(bar.querySelectorAll<HTMLElement>(`.${styles.item}`))
+}
+
+// NavLink's aria-current is the one record of which item is active.
+function activeItem(bar: HTMLElement): HTMLElement | null {
+  return bar.querySelector<HTMLElement>(`.${styles.item}[aria-current="page"]`)
+}
+
+// Puts the pill on the active item (hidden when no item is active) and
+// returns the resulting geometry.
+function place(bar: HTMLElement, pill: HTMLElement, active: HTMLElement | null): Geometry {
+  const geometry = {
+    active,
+    pill: { left: active?.offsetLeft ?? 0, width: active?.offsetWidth ?? 0 },
+    itemLefts: items(bar).map((item) => item.offsetLeft),
+  }
+  pill.hidden = !active
+  pill.style.left = `${geometry.pill.left}px`
+  pill.style.width = `${geometry.pill.width}px`
+  return geometry
+}