Add the swipe gesture to the Discover deck's top card

The top card follows a pointer drag per handoff Motion §4: captured with
setPointerCapture under touch-action none, it moves to translate(dx, dy*0.3)
rotate(dx/18 deg) with the "Try it!" and "Pass" stamps at clamp(±dx/100).
A release past 110 px right throws it from where it was let go (340 ms, no
wiggle); past 110 px left folds it from the drag transform; anything shorter
springs back over 420 ms and resets the stamps. Swipe decisions go through
the same decide handler as the buttons and ← / → keys, so the busy lock and
reduced-motion fade apply unchanged; only the top card is draggable and
input is ignored while busy.

motion.ts gains dragTransform, follow, unfollow and snapBack beside the
exits; DeckCard marks its Pass stamp as a part.

Maestro task 54c12e9e-5f36-4595-babb-e54107bb473b.

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

web/src/discover/DeckCard.tsx

@@ -1,3 +1,4 @@
+import type { DOMAttributes } from 'react'
 import { DIFFICULTIES, type Difficulty } from '../dinners/api.ts'
 import { Photo } from '../ui/Photo.tsx'
 import type { SideQuest } from './api.ts'
@@ -8,25 +9,32 @@ export const DECK_DEPTH = [0, 1, 2] as const
 
 const DIFFICULTY_LABEL: Record<Difficulty, string> = { easy: 'Easy', medium: 'Medium', hard: 'Hard' }
 
+// SwipeHandlers are the pointer handlers that drag the top card.
+export type SwipeHandlers = Pick<DOMAttributes<HTMLElement>, 'onPointerDown' | 'onPointerMove' | 'onPointerUp' | 'onPointerCancel'>
+
 export type DeckCardProps = {
   quest: SideQuest
   // 0 is the top card; 1 and 2 stack behind it.
   depth: number
+  // swipe makes the card draggable; only the top card gets it.
+  swipe?: SwipeHandlers
 }
 
 // DeckCard is one Side quest in the deck. Only the top card is read out and
 // reachable; the cards behind it are the stack's edge, kept rendered so they
 // can be promoted. Its "Try it!" and "Pass" stamps stay hidden until a
-// decision shows them, and its shade until Pass folds it (see motion.ts).
-export function DeckCard({ quest, depth }: DeckCardProps) {
+// decision or a drag shows them, and its shade until Pass folds it (see
+// motion.ts).
+export function DeckCard({ quest, depth, swipe }: DeckCardProps) {
   const top = depth === 0
   const titleId = `side-quest-${quest.id}`
   return (
     <article
-      className={`${styles.card} ${styles[`depth${depth}`]}`}
+      className={`${styles.card} ${styles[`depth${depth}`]}${swipe === undefined ? '' : ` ${styles.draggable}`}`}
       aria-labelledby={top ? titleId : undefined}
       aria-hidden={top ? undefined : true}
       inert={!top}
+      {...swipe}
     >
       <Photo id={quest.photo_id} name={quest.name} radius="24px" inset="10px" sizes="400px" className={styles.photo} />
       <div className={styles.content}>
@@ -41,7 +49,7 @@ export function DeckCard({ quest, depth }: DeckCardProps) {
       <span className={`${styles.stamp} ${styles.tryStamp}`} data-part="try-stamp" aria-hidden="true">
         Try it!
       </span>
-      <span className={`${styles.stamp} ${styles.passStamp}`} aria-hidden="true">
+      <span className={`${styles.stamp} ${styles.passStamp}`} data-part="pass-stamp" aria-hidden="true">
         Pass
       </span>
       <span className={styles.shade} data-part="shade" />

web/src/discover/DiscoverPage.module.css

@@ -104,6 +104,12 @@
   box-shadow: var(--shadow-lg);
 }
 
+/* The top card follows a drag instead of scrolling the page. */
+.draggable {
+  touch-action: none;
+  cursor: grab;
+}
+
 .depth1 {
   z-index: 2;
   transform: translateY(8px) scale(0.95);

web/src/discover/DiscoverPage.test.tsx

@@ -668,4 +668,342 @@ describe('DiscoverPage', () => {
       expect(sent(server.fetchMock)).toEqual([])
     })
   })
+
+  describe('swipe', () => {
+    const accepted = (quest: typeof BOWLS) => jsonResponse(200, { suggestion: { ...quest, status: 'accepted' }, replenish_job_id: null })
+    const declined = (quest: typeof BOWLS) => jsonResponse(200, { suggestion: { ...quest, status: 'declined' }, replenish_job_id: null })
+    const on = (animations: FakeAnimation[], target: Element) => animations.filter((a) => a.target === target)
+    const SPRING = 'cubic-bezier(.34,1.4,.64,1)'
+    let capture: Mock<(pointerId: number) => void>
+
+    // jsdom has no pointer capture.
+    beforeEach(() => {
+      capture = vi.fn()
+      Element.prototype.setPointerCapture = capture
+    })
+
+    afterEach(() => {
+      Reflect.deleteProperty(Element.prototype, 'setPointerCapture')
+    })
+
+    // press, drag and release put a pointer down on the card at (200, 300),
+    // move it by dx, dy, and let go.
+    function press(card: Element) {
+      fireEvent.pointerDown(card, { pointerId: 7, isPrimary: true, button: 0, clientX: 200, clientY: 300 })
+    }
+    function drag(card: Element, dx: number, dy: number) {
+      fireEvent.pointerMove(card, { pointerId: 7, isPrimary: true, clientX: 200 + dx, clientY: 300 + dy })
+    }
+    function release(card: Element) {
+      fireEvent.pointerUp(card, { pointerId: 7, isPrimary: true, button: 0 })
+    }
+    const stamps = (card: HTMLElement) => [within(card).getByText('Try it!'), within(card).getByText('Pass')]
+
+    it('makes the top card follow the pointer, captured, with the stamps showing how far it has gone', async () => {
+      stubServer([BOWLS, TIKKA])
+      const animations = stubAnimate({ autoFinish: false })
+      await renderPage()
+      const card = await screen.findByRole('article', { name: 'Korean beef bowls' })
+      const [tryStamp, passStamp] = stamps(card)
+      expect(declaredStyle(card).getPropertyValue('touch-action')).toBe('none')
+
+      press(card)
+      expect(capture).toHaveBeenCalledExactlyOnceWith(7)
+      expect(capture.mock.contexts[0]).toBe(card)
+
+      drag(card, 54, 20)
+      expect(card.style.transform).toBe('translate(54px,6px) rotate(3deg)')
+      expect(tryStamp!.style.opacity).toBe('0.54')
+      expect(passStamp!.style.opacity).toBe('0')
+
+      drag(card, -36, -10)
+      expect(card.style.transform).toBe('translate(-36px,-3px) rotate(-2deg)')
+      expect(tryStamp!.style.opacity).toBe('0')
+      expect(passStamp!.style.opacity).toBe('0.36')
+
+      drag(card, -180, 0)
+      expect(tryStamp!.style.opacity).toBe('0')
+      expect(passStamp!.style.opacity).toBe('1')
+      expect(animations).toEqual([])
+    })
+
+    it('Try it on a release past 110 px right: throws the card from where it was let go, in 340 ms with no wiggle, then records it', async () => {
+      const server = stubServer([BOWLS, TIKKA, ORZO], {
+        'POST /api/v1/suggestions/s1/accept': () => {
+          server.deck = [TIKKA, ORZO]
+          return accepted(BOWLS)
+        },
+      })
+      const animations = stubAnimate({ autoFinish: false })
+      await renderPage()
+      const card = await screen.findByRole('article', { name: 'Korean beef bowls' })
+      const pill = await screen.findByText('Quest log · 6')
+      const tryButton = screen.getByRole('button', { name: 'Try it' })
+
+      press(card)
+      drag(card, 126, 30)
+      release(card)
+      // No button bump, no wiggle: the stamp pops and the card flies.
+      expect(animations.map((a) => a.target)).toEqual([within(card).getByText('Try it!')])
+      await waitFor(() => expect(tryButton).toBeDisabled())
+      expect(animations).toHaveLength(2)
+      const fly = animations[1]!
+      expect(fly.target).toBe(card)
+      expect(fly.keyframes).toEqual([
+        { transform: 'translate(126px,9px) rotate(7deg)' },
+        { transform: 'translate(150%,-60px) rotate(28deg)', opacity: 0 },
+      ])
+      expect(fly.options).toEqual({ duration: 340, easing: 'cubic-bezier(.55,0,.8,.4)', fill: 'forwards' })
+      expect(sent(server.fetchMock)).toEqual([])
+
+      await finishRunning(animations)
+      expect(sent(server.fetchMock)).toEqual(['POST /api/v1/suggestions/s1/accept'])
+      expect(on(animations, pill)).toHaveLength(1)
+      const next = await screen.findByRole('article', { name: 'Crock pot chicken tikka masala' })
+      expect(on(animations, next).map((a) => a.options)).toEqual([{ duration: 300, easing: 'cubic-bezier(.2,.8,.2,1)' }])
+      await waitFor(() => expect(tryButton).toBeEnabled())
+      expect(next.style.transform).toBe('')
+    })
+
+    it('Pass on a release past 110 px left: folds the card from where it was let go, then records it', async () => {
+      const server = stubServer([BOWLS, TIKKA], {
+        'POST /api/v1/suggestions/s1/decline': () => {
+          server.deck = [TIKKA]
+          return declined(BOWLS)
+        },
+      })
+      const animations = stubAnimate({ autoFinish: false })
+      await renderPage()
+      const card = await screen.findByRole('article', { name: 'Korean beef bowls' })
+      const pill = await screen.findByText('Quest log · 6')
+      const shade = card.querySelector('[data-part="shade"]')!
+
+      press(card)
+      drag(card, -144, 20)
+      release(card)
+      expect(animations.map((a) => a.target)).toEqual([shade, card])
+      expect(animations[1]!.keyframes).toEqual([
+        { transformOrigin: '0% 50%', transform: 'translate(-144px,6px) rotate(-8deg) rotateY(0deg)', easing: 'cubic-bezier(.45,0,.85,.55)' },
+        { transformOrigin: '0% 50%', transform: 'translate(-144px,6px) rotate(-8deg) rotateY(-80deg)', offset: 0.45, easing: 'cubic-bezier(.4,0,.9,.6)' },
+        { transformOrigin: '0% 50%', transform: 'translate(-45%,560px) rotate(-35deg) rotateY(-90deg)', opacity: 0 },
+      ])
+      expect(animations[1]!.options).toEqual({ duration: 720, fill: 'forwards' })
+      await waitFor(() => expect(screen.getByRole('button', { name: 'Pass' })).toBeDisabled())
+      expect(sent(server.fetchMock)).toEqual([])
+
+      await finishRunning(animations)
+      expect(sent(server.fetchMock)).toEqual(['POST /api/v1/suggestions/s1/decline'])
+      await screen.findByRole('article', { name: 'Crock pot chicken tikka masala' })
+      expect(on(animations, pill)).toEqual([])
+    })
+
+    it.each([
+      ['right', 110],
+      ['left', -110],
+    ])('springs the card back from a release of 110 px %s, resetting the stamps and deciding nothing', async (_, dx) => {
+      const server = stubServer([BOWLS, TIKKA])
+      const animations = stubAnimate({ autoFinish: false })
+      await renderPage()
+      const card = await screen.findByRole('article', { name: 'Korean beef bowls' })
+      const [tryStamp, passStamp] = stamps(card)
+
+      press(card)
+      drag(card, dx, 0)
+      expect([tryStamp!.style.opacity, passStamp!.style.opacity]).toContain('1')
+      release(card)
+      expect(animations).toHaveLength(1)
+      expect(animations[0]!.target).toBe(card)
+      expect(animations[0]!.keyframes).toEqual([{ transform: `translate(${dx}px,0px) rotate(${dx / 18}deg)` }, { transform: 'translate(0px,0px) rotate(0deg)' }])
+      expect(animations[0]!.options).toEqual({ duration: 420, easing: SPRING })
+      expect(card.style.transform).toBe('')
+      expect(tryStamp!.style.opacity).toBe('')
+      expect(passStamp!.style.opacity).toBe('')
+      expect(getComputedStyle(tryStamp!).opacity).toBe('0')
+      expect(getComputedStyle(passStamp!).opacity).toBe('0')
+
+      // A later move with the pointer up does nothing.
+      drag(card, 200, 0)
+      expect(card.style.transform).toBe('')
+      await act(async () => {
+        await Promise.resolve()
+      })
+      expect(screen.getByRole('button', { name: 'Try it' })).toBeEnabled()
+      expect(sent(server.fetchMock)).toEqual([])
+    })
+
+    it('springs the card back when the pointer is canceled, however far it went', async () => {
+      const server = stubServer([BOWLS])
+      const animations = stubAnimate({ autoFinish: false })
+      await renderPage()
+      const card = await screen.findByRole('article', { name: 'Korean beef bowls' })
+
+      press(card)
+      drag(card, 180, 0)
+      fireEvent.pointerCancel(card, { pointerId: 7, isPrimary: true })
+      expect(animations.map((a) => a.options)).toEqual([{ duration: 420, easing: SPRING }])
+      expect(card.style.transform).toBe('')
+      expect(sent(server.fetchMock)).toEqual([])
+    })
+
+    it('ignores a drag while busy', async () => {
+      const server = stubServer([BOWLS, TIKKA], { 'POST /api/v1/suggestions/s1/decline': () => declined(BOWLS) })
+      const animations = stubAnimate({ autoFinish: false })
+      await renderPage()
+      const card = await screen.findByRole('article', { name: 'Korean beef bowls' })
+
+      fireEvent.click(screen.getByRole('button', { name: 'Pass' }))
+      const started = animations.length
+      press(card)
+      drag(card, 160, 0)
+      release(card)
+      expect(capture).not.toHaveBeenCalled()
+      expect(card.style.transform).toBe('')
+      expect(animations).toHaveLength(started)
+      await finishRunning(animations)
+      expect(sent(server.fetchMock)).toEqual(['POST /api/v1/suggestions/s1/decline'])
+    })
+
+    it('puts a drag held while a Retry goes out back at once on release, deciding nothing', async () => {
+      const retry = deferred()
+      let calls = 0
+      const server = stubServer([BOWLS, TIKKA], {
+        'POST /api/v1/suggestions/s1/accept': () => (++calls === 1 ? errorResponse(500, 'internal') : retry.promise),
+      })
+      await renderPage()
+      const card = await screen.findByRole('article', { name: 'Korean beef bowls' })
+      fireEvent.click(screen.getByRole('button', { name: 'Try it' }))
+      const alert = await screen.findByRole('alert')
+
+      press(card)
+      drag(card, 60, 0)
+      fireEvent.click(within(alert).getByRole('button', { name: 'Retry' }))
+      await waitFor(() => expect(screen.getByRole('button', { name: 'Try it' })).toBeDisabled())
+      drag(card, -160, 0)
+      expect(card.style.transform).toBe('translate(60px,0px) rotate(3.3333333333333335deg)')
+      const started = vi.mocked(Element.prototype.animate).mock.calls.length
+      release(card)
+      expect(card.style.transform).toBe('')
+      expect(within(card).getByText('Try it!').style.opacity).toBe('')
+      expect(vi.mocked(Element.prototype.animate).mock.calls.length).toBe(started)
+      expect(sent(server.fetchMock)).toEqual(['POST /api/v1/suggestions/s1/accept', 'POST /api/v1/suggestions/s1/accept'])
+      retry.resolve(accepted(BOWLS))
+      await waitFor(() => expect(screen.getByRole('button', { name: 'Try it' })).toBeEnabled())
+    })
+
+    it('drags only the top card', async () => {
+      const server = stubServer([BOWLS, TIKKA, ORZO])
+      const animations = stubAnimate({ autoFinish: false })
+      const { container } = await renderPage()
+      await screen.findByRole('article', { name: 'Korean beef bowls' })
+      const [, second, third] = container.querySelectorAll<HTMLElement>('article')
+
+      for (const card of [second!, third!]) {
+        expect(declaredStyle(card).getPropertyValue('touch-action')).toBe('')
+        press(card)
+        drag(card, 160, 0)
+        release(card)
+        expect(card.style.transform).toBe('')
+      }
+      expect(capture).not.toHaveBeenCalled()
+      expect(animations).toEqual([])
+      expect(sent(server.fetchMock)).toEqual([])
+    })
+
+    it('ignores a press that is not the primary button', async () => {
+      stubServer([BOWLS])
+      await renderPage()
+      const card = await screen.findByRole('article', { name: 'Korean beef bowls' })
+
+      fireEvent.pointerDown(card, { pointerId: 7, isPrimary: true, button: 2, clientX: 200, clientY: 300 })
+      drag(card, 160, 0)
+      expect(capture).not.toHaveBeenCalled()
+      expect(card.style.transform).toBe('')
+    })
+
+    it('a key decision mid-drag ends the drag and throws the card from rest', async () => {
+      const server = stubServer([BOWLS, TIKKA], {
+        'POST /api/v1/suggestions/s1/accept': () => {
+          server.deck = [TIKKA]
+          return accepted(BOWLS)
+        },
+      })
+      const animations = stubAnimate({ autoFinish: false })
+      await renderPage()
+      const card = await screen.findByRole('article', { name: 'Korean beef bowls' })
+      const [tryStamp] = stamps(card)
+
+      press(card)
+      drag(card, -60, 0)
+      fireEvent.keyDown(window, { key: 'ArrowRight' })
+      expect(card.style.transform).toBe('')
+      expect(tryStamp!.style.opacity).toBe('')
+      // The button bump, stamp pop and wiggle of a Try it from rest.
+      expect(animations.map((a) => a.target)).toEqual([screen.getByRole('button', { name: 'Try it' }), tryStamp, card])
+      release(card)
+      expect(animations).toHaveLength(3)
+      await finishRunning(animations)
+      await finishRunning(animations)
+      expect(sent(server.fetchMock)).toEqual(['POST /api/v1/suggestions/s1/accept'])
+    })
+
+    it('puts a swiped card back at rest when its decision is refused', async () => {
+      stubServer([BOWLS], { 'POST /api/v1/suggestions/s1/decline': () => errorResponse(500, 'internal') })
+      const animations = stubAnimate({ autoFinish: false })
+      await renderPage()
+      const card = await screen.findByRole('article', { name: 'Korean beef bowls' })
+      const [tryStamp, passStamp] = stamps(card)
+
+      press(card)
+      drag(card, -150, 0)
+      release(card)
+      await finishRunning(animations)
+      await screen.findByRole('alert')
+      expect(animations.map((a) => a.playState)).toEqual(['idle', 'idle'])
+      expect(card.style.transform).toBe('')
+      expect(tryStamp!.style.opacity).toBe('')
+      expect(passStamp!.style.opacity).toBe('')
+      expect(screen.getByRole('button', { name: 'Pass' })).toBeEnabled()
+    })
+
+    it.each([
+      ['Try it', 140, 'accept'],
+      ['Pass', -140, 'decline'],
+    ] as const)('%s by swipe with reduced motion fades the card where it was let go', async (_, dx, action) => {
+      mockViewport(390, { reducedMotion: true })
+      const server = stubServer([BOWLS, TIKKA], {
+        [`POST /api/v1/suggestions/s1/${action}`]: () => {
+          server.deck = [TIKKA]
+          return action === 'accept' ? accepted(BOWLS) : declined(BOWLS)
+        },
+      })
+      const animations = stubAnimate({ autoFinish: false })
+      await renderPage()
+      const card = await screen.findByRole('article', { name: 'Korean beef bowls' })
+
+      press(card)
+      drag(card, dx, 0)
+      release(card)
+      expect(animations.map((a) => [a.target, a.keyframes, a.options])).toEqual([[card, [{ opacity: 1 }, { opacity: 0 }], { duration: 200, fill: 'forwards' }]])
+      expect(card.style.transform).toBe(`translate(${dx}px,0px) rotate(${dx / 18}deg)`)
+      await finishRunning(animations)
+      expect(sent(server.fetchMock)).toEqual([`POST /api/v1/suggestions/s1/${action}`])
+      await screen.findByRole('article', { name: 'Crock pot chicken tikka masala' })
+      expect(animations).toHaveLength(1)
+    })
+
+    it('puts the card back at once, without the spring, from a short swipe with reduced motion', async () => {
+      mockViewport(390, { reducedMotion: true })
+      stubServer([BOWLS])
+      const animations = stubAnimate({ autoFinish: false })
+      await renderPage()
+      const card = await screen.findByRole('article', { name: 'Korean beef bowls' })
+
+      press(card)
+      drag(card, 80, 0)
+      release(card)
+      expect(animations).toEqual([])
+      expect(card.style.transform).toBe('')
+      expect(within(card).getByText('Try it!').style.opacity).toBe('')
+    })
+  })
 })

web/src/discover/DiscoverPage.tsx

@@ -18,10 +18,30 @@ import {
   useStartBatch,
   type Choice,
 } from './api.ts'
-import { DeckCard, DECK_DEPTH } from './DeckCard.tsx'
-import { bump, fadeOut, isAbort, pass, prefersReducedMotion, promote, tryIt, type Exit } from './motion.ts'
+import { DeckCard, DECK_DEPTH, type SwipeHandlers } from './DeckCard.tsx'
+import {
+  bump,
+  fadeOut,
+  follow,
+  isAbort,
+  pass,
+  prefersReducedMotion,
+  promote,
+  snapBack,
+  tryIt,
+  unfollow,
+  type Exit,
+} from './motion.ts'
 import styles from './DiscoverPage.module.css'
 
+// SWIPE_THRESHOLD is how far, in px, the top card must be dragged left or
+// right for its release to Pass or Try it.
+const SWIPE_THRESHOLD = 110
+
+// A Drag is the pointer dragging the top card: where it went down, and the
+// card's drag transform so far.
+type Drag = { pointerId: number; x: number; y: number; dx: number; transform: string | null }
+
 // isEditable says whether a key press belongs to a field being typed in,
 // which the deck's ← / → keys leave alone.
 function isEditable(target: EventTarget | null): boolean {
@@ -66,12 +86,12 @@ function QuestLogPill({ dinners, ref }: { dinners: { id: string; name: string }[
 }
 
 // DiscoverPage is mock 1a: the deck of Side quests to Try it or Pass, one
-// at a time from the top card. A decision from the buttons or the ← / → keys
-// throws the top card out (motion.ts) and sends the decision once it is
-// gone; Retry sends a refused one again. Input is locked from the press
-// until the decision settles: the lock is a ref, set before anything
-// starts, so a second press before the next render does nothing, and busy
-// mirrors it to disable the buttons.
+// at a time from the top card. A decision from the buttons, the ← / → keys
+// or a swipe of the top card throws it out (motion.ts) and sends the
+// decision once it is gone; Retry sends a refused one again. Input is
+// locked from the press until the decision settles: the lock is a ref, set
+// before anything starts, so a second press before the next render does
+// nothing, and busy mirrors it to disable the buttons.
 export function DiscoverPage() {
   const queryClient = useQueryClient()
   const deck = useDeck()
@@ -106,6 +126,7 @@ export function DiscoverPage() {
   // Side quest the last decision sent was for, until the deck moves past it.
   const exit = useRef<Exit | null>(null)
   const leaving = useRef<string | null>(null)
+  const drag = useRef<Drag | null>(null)
 
   const { mutate: mutateDecision } = decision
   const deciding = useRef(false)
@@ -141,17 +162,37 @@ export function DiscoverPage() {
   )
   // decide bumps the button, throws the top card out — or, motion reduced,
   // fades it — then sends the decision; Try it bumps the Quest log pill too.
+  // A swipe's decision comes from the card's drag transform: it is thrown
+  // from there without a bump or wiggle. A button or key decision ends a
+  // drag in progress, putting the card back first.
   const decide = useCallback(
-    (choice: Choice) => {
+    (choice: Choice, from?: string) => {
       if (top === undefined || !lock()) return
       const card = deckRef.current!.querySelector('article')!
+      if (from === undefined && drag.current !== null) {
+        drag.current = null
+        unfollow(card)
+      }
       const reduced = prefersReducedMotion()
       let run: Exit
       if (reduced) {
         run = fadeOut(card)
-      } else {
+      } else if (from === undefined) {
         bump((choice === 'try' ? tryRef : passRef).current!)
         run = choice === 'try' ? tryIt(card) : pass(card)
+      } else {
+        run = choice === 'try' ? tryIt(card, from) : pass(card, from)
+      }
+      if (from !== undefined) {
+        // Putting a swiped card back puts it back at rest.
+        const thrown = run
+        run = {
+          finished: thrown.finished,
+          cancel: () => {
+            thrown.cancel()
+            unfollow(card)
+          },
+        }
       }
       exit.current = run
       run.finished.then(
@@ -168,6 +209,39 @@ export function DiscoverPage() {
     [top, lock, send],
   )
 
+  // swipe drags the top card: it follows the pointer, captured from the
+  // press, until the release decides past the threshold or springs it back
+  // short of it. Input while busy is ignored, and a drag held into a busy
+  // spell (a Retry) is put back on release.
+  const swipe = useMemo<SwipeHandlers>(() => {
+    function release(card: HTMLElement, pointerId: number, decides: boolean) {
+      const current = drag.current
+      if (current === null || current.pointerId !== pointerId) return
+      drag.current = null
+      if (current.transform === null) return
+      if (deciding.current) unfollow(card)
+      else if (decides && current.dx > SWIPE_THRESHOLD) decide('try', current.transform)
+      else if (decides && current.dx < -SWIPE_THRESHOLD) decide('pass', current.transform)
+      else if (prefersReducedMotion()) unfollow(card)
+      else snapBack(card, current.transform)
+    }
+    return {
+      onPointerDown: (event) => {
+        if (deciding.current || !event.isPrimary || event.button !== 0) return
+        event.currentTarget.setPointerCapture(event.pointerId)
+        drag.current = { pointerId: event.pointerId, x: event.clientX, y: event.clientY, dx: 0, transform: null }
+      },
+      onPointerMove: (event) => {
+        const current = drag.current
+        if (current === null || current.pointerId !== event.pointerId || deciding.current) return
+        current.dx = event.clientX - current.x
+        current.transform = follow(event.currentTarget, current.dx, event.clientY - current.y)
+      },
+      onPointerUp: (event) => release(event.currentTarget, event.pointerId, true),
+      onPointerCancel: (event) => release(event.currentTarget, event.pointerId, false),
+    }
+  }, [decide])
+
   // Leaving the page stops an exit before its decision is sent.
   useEffect(() => () => exit.current?.cancel(), [])
 
@@ -239,7 +313,7 @@ export function DiscoverPage() {
       <>
         <div ref={deckRef} className={styles.deck}>
           {deck.data.slice(0, DECK_DEPTH.length).map((quest, depth) => (
-            <DeckCard key={quest.id} quest={quest} depth={depth} />
+            <DeckCard key={quest.id} quest={quest} depth={depth} swipe={depth === 0 ? swipe : undefined} />
           ))}
         </div>
         <div className={styles.actions}>

web/src/discover/motion.test.ts

@@ -1,6 +1,6 @@
 import { afterEach, beforeEach, describe, expect, it } from 'vitest'
 import { stubAnimate, type FakeAnimation } from '../test/animate.ts'
-import { bump, fadeOut, isAbort, pass, promote, tryIt } from './motion.ts'
+import { bump, dragTransform, fadeOut, follow, isAbort, pass, promote, snapBack, tryIt, unfollow } from './motion.ts'
 
 let animations: FakeAnimation[]
 
@@ -15,10 +15,11 @@ afterEach(() => {
 // card builds a deck card with the parts DeckCard marks.
 function card() {
   const article = document.createElement('article')
-  article.innerHTML = '<span data-part="try-stamp"></span><span data-part="shade"></span>'
+  article.innerHTML = '<span data-part="try-stamp"></span><span data-part="pass-stamp"></span><span data-part="shade"></span>'
   return {
     article,
-    stamp: article.querySelector('[data-part="try-stamp"]')!,
+    stamp: article.querySelector<HTMLElement>('[data-part="try-stamp"]')!,
+    passStamp: article.querySelector<HTMLElement>('[data-part="pass-stamp"]')!,
     shade: article.querySelector('[data-part="shade"]')!,
   }
 }
@@ -169,8 +170,52 @@ describe('motion', () => {
     expect(animations.map((a) => a.playState)).toEqual(['idle', 'idle'])
   })
 
+  it('drags: translate(dx, dy·0.3) rotate(dx/18 deg)', () => {
+    expect(dragTransform(90, 20)).toBe('translate(90px,6px) rotate(5deg)')
+    expect(dragTransform(-36, -10)).toBe('translate(-36px,-3px) rotate(-2deg)')
+  })
+
+  it.each([
+    [50, '0.5', '0'],
+    [-50, '0', '0.5'],
+    [160, '1', '0'],
+    [-160, '0', '1'],
+    [0, '0', '0'],
+  ])('follows a drag of %i px with the stamps at Try it! %s, Pass %s', (dx, tryOpacity, passOpacity) => {
+    const { article, stamp, passStamp } = card()
+    expect(follow(article, dx, 0)).toBe(dragTransform(dx, 0))
+    expect(article.style.transform).toBe(dragTransform(dx, 0))
+    expect(stamp.style.opacity).toBe(tryOpacity)
+    expect(passStamp.style.opacity).toBe(passOpacity)
+    expect(animations).toEqual([])
+  })
+
+  it('unfollows at once, back to the stylesheet', () => {
+    const { article, stamp, passStamp } = card()
+    follow(article, -80, 12)
+    unfollow(article)
+    expect(article.getAttribute('style')).toBe('')
+    expect(stamp.getAttribute('style')).toBe('')
+    expect(passStamp.getAttribute('style')).toBe('')
+    expect(animations).toEqual([])
+  })
+
+  it('snaps back from the drag transform to rest, 420 ms spring, and hides the stamps', () => {
+    const { article, stamp, passStamp } = card()
+    const from = follow(article, 70, 30)
+    snapBack(article, from)
+    expect(article.style.transform).toBe('')
+    expect(stamp.style.opacity).toBe('')
+    expect(passStamp.style.opacity).toBe('')
+    expect(animations).toHaveLength(1)
+    expect(animations[0]!.target).toBe(article)
+    expect(animations[0]!.keyframes).toEqual([{ transform: 'translate(70px,9px) rotate(3.888888888888889deg)' }, { transform: 'translate(0px,0px) rotate(0deg)' }])
+    expect(animations[0]!.options).toEqual({ duration: 420, easing: 'cubic-bezier(.34,1.4,.64,1)' })
+  })
+
   it('fails fast on a card without its parts', () => {
     expect(() => tryIt(document.createElement('article'))).toThrow('deck card has no try-stamp')
     expect(() => pass(document.createElement('article'))).toThrow('deck card has no shade')
+    expect(() => follow(document.createElement('article'), 10, 0)).toThrow('deck card has no try-stamp')
   })
 })

web/src/discover/motion.ts

@@ -1,7 +1,7 @@
 // The deck's motion, from the handoff's Motion §2 (Try it), §3 (Pass),
-// §5 (deck advance) and §6 (reduced motion), with the Web Animations API.
-// The exits take the card's starting transform so the swipe gesture can
-// throw a dragged card from where it was let go.
+// §4 (swipe), §5 (deck advance) and §6 (reduced motion), with the Web
+// Animations API. The exits take the card's starting transform so the swipe
+// gesture can throw a dragged card from where it was let go.
 
 const SPRING = 'cubic-bezier(.34,1.4,.64,1)'
 const THROW = 'cubic-bezier(.55,0,.8,.4)'
@@ -93,6 +93,38 @@ export function pass(card: HTMLElement, from: string = REST): Exit {
   return exit.done(finished)
 }
 
+// dragTransform is the top card's transform dragged dx, dy from where the
+// pointer went down.
+export function dragTransform(dx: number, dy: number): string {
+  return `translate(${dx}px,${dy * 0.3}px) rotate(${dx / 18}deg)`
+}
+
+// follow moves a dragged card to its drag transform and shows each stamp as
+// far as the drag has gone toward its side, fully at 100 px. It returns the
+// transform, for the exit or snap-back the release starts.
+export function follow(card: HTMLElement, dx: number, dy: number): string {
+  const transform = dragTransform(dx, dy)
+  card.style.transform = transform
+  part(card, 'try-stamp').style.opacity = String(clamp(dx / 100))
+  part(card, 'pass-stamp').style.opacity = String(clamp(-dx / 100))
+  return transform
+}
+
+// unfollow puts a dragged card and its stamps back where the stylesheet
+// places them, at once.
+export function unfollow(card: HTMLElement) {
+  card.style.removeProperty('transform')
+  part(card, 'try-stamp').style.removeProperty('opacity')
+  part(card, 'pass-stamp').style.removeProperty('opacity')
+}
+
+// snapBack springs a card let go short of a decision back to rest from its
+// drag transform, and hides its stamps.
+export function snapBack(card: HTMLElement, from: string): Animation {
+  unfollow(card)
+  return card.animate([{ transform: from }, { transform: REST }], { duration: 420, easing: SPRING })
+}
+
 // fadeOut is both exits under prefers-reduced-motion: reduce.
 export function fadeOut(card: HTMLElement): Exit {
   const exit = recorder()
@@ -110,6 +142,10 @@ export function promote(cards: HTMLElement[]): Animation[] {
   )
 }
 
+function clamp(value: number): number {
+  return Math.min(1, Math.max(0, value))
+}
+
 // part is the card's element marked data-part="name", which DeckCard renders.
 function part(card: HTMLElement, name: string): HTMLElement {
   const element = card.querySelector<HTMLElement>(`[data-part="${name}"]`)