@Rekoo-PS reported zoom on mobile only shows the centre crop — the
image zooms but you can't pan around to see other parts. Single-finger
touch was being routed through the carousel-swipe branch which is
gated on zoom <= 1 (so swipe doesn't fight with pan), so when zoomed
the touch hit no handler at all.
Desktop has the equivalent path via handleMouseDown / handleMouseMove
(line 364), which is why this only manifests on mobile.
Add a single-finger pan branch to the touch handlers that mirrors the
mouse path:
- handleTouchStart: when zoom > 1 and one finger, record dragStart
relative to the existing dragOffset (so subsequent moves continue
from where the last pan left off, not from origin).
- handleTouchMove: when isDragging + zoom > 1 + one finger, update
dragOffset from touch position.
- handleTouchEnd: clear the isDragging flag (offset persists so the
image stays where the user left it).
Also fix a latent bug surfaced while reading the pinch-zoom path:
when pinch-out drops zoom back to 1.0, dragOffset wasn't reset, so the
photo sat off-centre at natural zoom. Re-centre in handleTouchMove
when newZoom drops to <=1 with a non-zero offset.
Carousel swipe stays disabled when zoomed (existing behaviour). Mouse
path untouched. Pinch-to-zoom path untouched.
Refs: #532
Paul Nothaft
53139b8cb8