Two fixes for discussion #348.
Carousel-style swipe
The lightbox previously snapped to the next photo on swipe, then showed
a loading spinner while the new image fetched — choppy compared with
the reference video the reporter shared. The current photo is now
rendered inside a 3-slide track (prev/current/next). As the finger
drags, the track follows; on release the track animates to the
neighbouring slot or springs back if the gesture didn't pass the
threshold. Because the prev/next AuthenticatedImages render up front,
the browser starts fetching them while the user is still on the
current photo, so there's no loader flash on commit.
- Phase machine ('idle' | 'dragging' | 'committing' | 'springing')
drives the track's transform/transition. Commit + spring use a 280ms
cubic-bezier ease.
- Percentage-based transforms avoid measuring container width before
the first paint. Commit threshold (read from the ref on demand) is
max(60px, 20% of width) OR a fast flick (>0.5 px/ms with at least
40px of movement).
- transitionend advances currentIndex with wrap-around and resets the
track in one batch — slot contents rotate and the track snaps from
the commit position back to centered with transition: none, so the
visible image stays put. No flicker.
- Vertical-cancel (>24px dy) abandons the drag and springs back so the
user keeps the gesture they intended.
- touch-action: none on the carousel container stops the browser
fighting us with edge-swipe back navigation and native pinch-zoom.
- Pinch starting mid-drag springs the track back smoothly so the image
doesn't jerk under the second finger.
- onTouchCancel covers system-interrupted gestures (incoming call etc).
- dragX === 0 short-circuits to 'idle' instead of 'springing' so taps
don't get stuck waiting for a transitionend that never fires.
- Neighbour slides use a simplified AuthenticatedImage render (no
canvas/fragment-grid pipeline) since they're only on screen during
the swipe; the current slide keeps the full protection chain.
- Neighbour videos render their thumbnail rather than spinning up a
VideoPlayer. When the *current* photo is a video, the carousel is
bypassed entirely — single VideoPlayer + no swipe handlers — because
sliding a video element during a drag is awkward and adds nothing.
- Removed the now-redundant imageLoaded state + spinner;
AuthenticatedImage already shows a placeholder while loading.
Keyboard arrows and the on-screen Prev/Next buttons still snap (no
animation) — animating them would have required input queuing for
fast double-presses, and the request was specifically about swipe.
"Swipe to navigate" hint
Removed the mobile-only overlay text. Swipe is universal in image
viewers; the instruction read like training wheels and competed with
the photo for attention.
Paul Nothaft
743086d3cb