fix: the image viewer pans without unzooming, scrolls with the wheel, and opens a long strip at full width

- The click that ends a drag no longer resets the zoom.
- The wheel scrolls a zoomed image (shift+wheel goes sideways). Ctrl+wheel or
  a trackpad pinch zooms at the pointer.
- Clicking a long strip or panorama zooms it to fill the canvas, so a comic
  strip opens at reading width. Anything else zooms 2x. The clicked point
  stays under the cursor.
- The zoom limit follows the image's own resolution instead of a flat 4x.
- Panning stops at the image's edges.
- A zoom bar (−, slider, +, size against full resolution, fit) fades in at
  the bottom of the canvas, and the shortcut sheet lists the gestures.

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

frontend/src/components/modal/ImageCanvas.vue

@@ -2,11 +2,15 @@
   <div
     ref="canvasEl"
     class="fc-canvas"
-    :class="{ 'fc-canvas--zoomed': panZoom.state.scale > 1 }"
+    :class="{
+      'fc-canvas--zoomed': panZoom.state.scale > 1,
+      'fc-canvas--moving': !panZoom.state.smooth,
+    }"
     @wheel="panZoom.handlers.onWheel"
     @pointerdown="panZoom.handlers.onPointerDown"
     @pointermove="panZoom.handlers.onPointerMove"
     @pointerup="panZoom.handlers.onPointerUp"
+    @pointercancel="panZoom.handlers.onPointerCancel"
     @click="onCanvasClick"
   >
     <img
@@ -18,6 +22,34 @@
       draggable="false"
       @load="onImgLoad"
     >
+    <!-- Zoom controls. They sit on the canvas but are not part of it: a press,
+         click or scroll on them never pans, toggles or closes. -->
+    <div
+      class="fc-canvas__zoombar"
+      @pointerdown.stop @click.stop @wheel.stop
+    >
+      <button
+        type="button" class="fc-canvas__zoombtn" aria-label="Zoom out"
+        :disabled="panZoom.state.scale <= 1" @click="panZoom.zoomBy(-1)"
+      ><v-icon size="18">mdi-minus</v-icon></button>
+      <input
+        type="range" class="fc-canvas__zoomslider" aria-label="Zoom"
+        min="0" max="1" step="0.001" :value="sliderValue"
+        @input="onSlider"
+      >
+      <button
+        type="button" class="fc-canvas__zoombtn" aria-label="Zoom in"
+        :disabled="panZoom.state.scale >= maxScale" @click="panZoom.zoomBy(1)"
+      ><v-icon size="18">mdi-plus</v-icon></button>
+      <span class="fc-canvas__zoomlabel" :title="'Shown at ' + zoomLabel + ' of its full size'">
+        {{ zoomLabel }}
+      </span>
+      <button
+        type="button" class="fc-canvas__zoombtn" aria-label="Fit to the window"
+        title="Fit to the window"
+        :disabled="panZoom.state.scale <= 1" @click="panZoom.reset()"
+      ><v-icon size="18">mdi-fit-to-screen-outline</v-icon></button>
+    </div>
     <!-- #1206 grounding overlay: tracks the <img>'s live rendered rect (so it's
          correct under object-fit letterboxing + pan/zoom) and draws the crop
          region a hovered suggestion came from. Null grounding → a subtle
@@ -60,10 +92,37 @@ const props = defineProps({
   hover: { type: Object, default: null },
 })
 const emit = defineEmits(['close-request'])
-const panZoom = usePanZoom()
-
 const canvasEl = ref(null)
 const imgEl = ref(null)
+// Where the picture sits before any zoom: the canvas size, the picture's
+// layout box (the zoom is a transform on top of it) and its natural width.
+// Read again when the picture loads and when the window resizes.
+const fitted = ref(null)
+function refit() {
+  const img = imgEl.value
+  const canvas = canvasEl.value
+  fitted.value = img && canvas ? {
+    cw: canvas.clientWidth, ch: canvas.clientHeight,
+    iw: img.offsetWidth, ih: img.offsetHeight, nw: img.naturalWidth,
+  } : null
+  panZoom.clamp()
+}
+const panZoom = usePanZoom(() => fitted.value)
+
+// The slider runs on a log scale, so each stretch of it is the same zoom step
+// whether the picture is at 1.2x or 12x.
+const maxScale = computed(() => panZoom.maxScale())
+const sliderValue = computed(() =>
+  Math.log(panZoom.state.scale) / Math.log(maxScale.value))
+function onSlider(ev) {
+  panZoom.setScale(maxScale.value ** Number(ev.target.value))
+}
+// How big it is against its full-size pixels.
+const zoomLabel = computed(() => {
+  const g = fitted.value
+  const pct = g?.iw && g?.nw ? (g.iw * panZoom.state.scale) / g.nw : panZoom.state.scale
+  return `${Math.round(pct * 100)}%`
+})
 const overlayBox = ref(null)   // the <img>'s rect relative to the canvas
 
 // The <img> renders aspect-fit + transformed, so its getBoundingClientRect is
@@ -107,22 +166,21 @@ watch(
   () => [panZoom.state.scale, panZoom.state.x, panZoom.state.y],
   () => { if (props.hover) measure() },
 )
-function onResize () { if (props.hover) measure() }
-function onImgLoad () { if (props.hover) measure() }
+function onResize () { refit(); if (props.hover) measure() }
+function onImgLoad () { refit(); if (props.hover) measure() }
 onMounted(() => window.addEventListener('resize', onResize))
 onUnmounted(() => window.removeEventListener('resize', onResize))
 
 // Reset zoom when the src changes (prev/next nav).
 watch(() => props.src, () => panZoom.reset())
 
-// Click on the image → toggle zoom (IR/expected behavior). Click on
-// the haze area around the image → request close. We use the image's
-// own bounding rect rather than @click.self because the <img> sets
-// pointer-events: none for the pan-drag pathway, so the click always
-// targets the canvas div regardless of where the cursor was.
+// Click on the image → toggle zoom. Click on the haze around it → request
+// close. The <img> has pointer-events: none so a drag always lands on the
+// canvas, which is why "on the image" is worked out from its rect. The click
+// that ends a drag does neither.
 function onCanvasClick(ev) {
+  if (panZoom.endedDrag()) return
   if (panZoom.state.scale > 1) {
-    // When zoomed, any click resets zoom — preserve old behavior.
     panZoom.handlers.onClick(ev)
     return
   }
@@ -160,6 +218,49 @@ function onCanvasClick(ev) {
   transition: transform 100ms ease-out;
   pointer-events: none;
 }
+.fc-canvas--moving img { transition: none; }
+
+/* Zoom controls, bottom centre of the canvas: quiet until the canvas is
+   hovered or the picture is zoomed. */
+.fc-canvas__zoombar {
+  position: absolute;
+  bottom: 16px; left: 50%;
+  transform: translateX(-50%);
+  z-index: 3;
+  display: flex; align-items: center; gap: 4px;
+  padding: 4px 8px;
+  border-radius: 999px;
+  background: rgba(var(--v-theme-background), 0.7);
+  border: 1px solid rgb(var(--v-theme-surface-light));
+  color: rgb(var(--v-theme-on-surface));
+  cursor: default;
+  opacity: 0;
+  transition: opacity 150ms ease;
+}
+.fc-canvas:hover .fc-canvas__zoombar,
+.fc-canvas--zoomed .fc-canvas__zoombar,
+.fc-canvas__zoombar:focus-within { opacity: 1; }
+.fc-canvas__zoombtn {
+  width: 28px; height: 28px;
+  display: grid; place-items: center;
+  border-radius: 50%;
+  color: inherit;
+  cursor: pointer;
+}
+.fc-canvas__zoombtn:hover:not(:disabled) { background: rgb(var(--v-theme-surface)); }
+.fc-canvas__zoombtn:disabled { opacity: 0.35; cursor: default; }
+.fc-canvas__zoomslider {
+  width: 140px;
+  accent-color: rgb(var(--v-theme-accent));
+  cursor: pointer;
+}
+.fc-canvas__zoomlabel {
+  min-width: 44px;
+  text-align: center;
+  font-size: 12px;
+  font-variant-numeric: tabular-nums;
+  color: rgb(var(--v-theme-on-surface-variant));
+}
 
 /* Grounding overlay — never intercepts pan/zoom/click. */
 .fc-canvas__overlay {

frontend/src/components/modal/ImageViewer.vue

@@ -18,6 +18,9 @@
           <h3>Keyboard shortcuts</h3>
           <dl>
             <div><dt>← / →</dt><dd>Previous / next image</dd></div>
+            <div><dt>Click</dt><dd>Zoom in (a long strip opens at full width) / back to fit</dd></div>
+            <div><dt>Scroll · drag</dt><dd>Move around a zoomed image</dd></div>
+            <div><dt>Ctrl + scroll</dt><dd>Zoom at the pointer</dd></div>
             <div><dt>T or /</dt><dd>Jump to the tag input</dd></div>
             <div><dt>↑ / ↓</dt><dd>Move through tag suggestions</dd></div>
             <div><dt>Enter / Tab</dt><dd>Accept the highlighted tag</dd></div>

frontend/src/composables/usePanZoom.js

@@ -1,72 +1,174 @@
-// Click-to-toggle zoom + drag-to-pan when zoomed + wheel-to-zoom.
-// Returns reactive state + handlers a component can spread onto its
-// root element.
+// Zoom and pan for one picture in a canvas.
+//
+// - Click toggles between the fitted view and a zoomed one. A long strip (or a
+//   wide panorama) zooms to fill the canvas across its short side, so a comic
+//   strip opens at reading width; anything else zooms to 2x. The clicked point
+//   stays under the cursor.
+// - A drag pans, and the click that ends a drag does not toggle the zoom.
+// - The wheel scrolls the zoomed picture (shift+wheel goes sideways); with
+//   ctrl held, which is also what a trackpad pinch sends, it zooms at the
+//   cursor.
+// - zoomBy/setScale drive the toolbar's buttons and slider.
+//
+// `measure()` returns the canvas size, the picture's fitted (unzoomed) size
+// and its natural width: { cw, ch, iw, ih, nw }, or null before it has laid
+// out. The picture is centred in the canvas and scaled about its centre, so a
+// translate of (x, y) at scale s puts picture point p at centre + (x, y) + s*p.
 
 import { reactive } from 'vue'
 
-const MIN_SCALE = 1
-const MAX_SCALE = 4
-const WHEEL_SENSITIVITY = 0.0015
+export const MIN_SCALE = 1
+const CLICK_SCALE = 2
+// A picture that would have to grow more than this to fill the canvas is a
+// strip or a panorama: its click zoom fills the canvas instead of doubling.
+const LONG_FILL = 2
+const MAX_FLOOR = 4      // the most you can zoom a small picture
+const MAX_CEIL = 32
+const STEP = 1.25        // one press of + or -
+const DRAG_PX = 4        // movement before a press counts as a drag
+const PINCH_SENSITIVITY = 0.01
+const LINE_PX = 40       // a wheel "line" (deltaMode 1)
 
-export function usePanZoom() {
+export function usePanZoom(measure = () => null) {
   const state = reactive({
     scale: 1,
     x: 0,
     y: 0,
-    panning: false
+    panning: false,
+    // Off while dragging or scrolling, so the picture follows the hand
+    // instead of easing behind it.
+    smooth: true,
   })
 
-  let dragOriginX = 0
-  let dragOriginY = 0
-  let panOriginX = 0
-  let panOriginY = 0
+  let press = null       // { x, y, panX, panY, id } while a button is down
+  let dragged = false    // the press that is ending moved: swallow its click
+
+  function geometry() {
+    const g = measure()
+    return g && g.iw > 0 && g.ih > 0 && g.cw > 0 && g.ch > 0 ? g : null
+  }
+
+  // The scale at which the picture covers the canvas both ways.
+  function fillScale(g = geometry()) {
+    return g ? Math.max(g.cw / g.iw, g.ch / g.ih) : CLICK_SCALE
+  }
+
+  function maxScale(g = geometry()) {
+    if (!g) return MAX_FLOOR
+    const actualPixels = g.nw ? g.nw / g.iw : 0
+    return Math.min(MAX_CEIL, Math.max(MAX_FLOOR, fillScale(g), actualPixels * 2))
+  }
+
+  function clickScale(g = geometry()) {
+    const fill = fillScale(g)
+    return Math.min(maxScale(g), fill > LONG_FILL ? fill : CLICK_SCALE)
+  }
+
+  // Keep the picture over the canvas: no panning past its edges, and a side
+  // that is smaller than the canvas stays centred.
+  function clamp() {
+    const g = geometry()
+    if (!g) return
+    const mx = Math.max(0, (g.iw * state.scale - g.cw) / 2)
+    const my = Math.max(0, (g.ih * state.scale - g.ch) / 2)
+    state.x = Math.min(mx, Math.max(-mx, state.x))
+    state.y = Math.min(my, Math.max(-my, state.y))
+  }
 
   function reset() {
+    state.smooth = true
     state.scale = 1
     state.x = 0
     state.y = 0
   }
 
+  // Zoom to `scale` keeping the point (px, py), measured from the canvas
+  // centre, where it is on screen.
+  function zoomAt(scale, px = 0, py = 0) {
+    const next = Math.min(maxScale(), Math.max(MIN_SCALE, scale))
+    if (next === MIN_SCALE) { state.scale = next; state.x = 0; state.y = 0; return }
+    const k = next / state.scale
+    state.x = px - (px - state.x) * k
+    state.y = py - (py - state.y) * k
+    state.scale = next
+    clamp()
+  }
+
+  function setScale(scale) {
+    state.smooth = true
+    zoomAt(scale)
+  }
+
+  function zoomBy(steps) {
+    setScale(state.scale * STEP ** steps)
+  }
+
+  function fromCentre(ev) {
+    const r = ev.currentTarget?.getBoundingClientRect?.()
+    if (!r) return [0, 0]
+    return [ev.clientX - r.left - r.width / 2, ev.clientY - r.top - r.height / 2]
+  }
+
   function toggleZoom(ev) {
-    if (state.scale > 1) { reset(); return }
-    state.scale = 2
-    // Zoom centered on click point
-    if (ev && ev.currentTarget) {
-      const rect = ev.currentTarget.getBoundingClientRect()
-      const cx = ev.clientX - rect.left - rect.width / 2
-      const cy = ev.clientY - rect.top - rect.height / 2
-      state.x = -cx
-      state.y = -cy
-    }
+    if (state.scale > MIN_SCALE) { reset(); return }
+    state.smooth = true
+    zoomAt(clickScale(), ...(ev ? fromCentre(ev) : [0, 0]))
   }
 
   function onWheel(ev) {
     ev.preventDefault()
-    const delta = -ev.deltaY * WHEEL_SENSITIVITY
-    const next = Math.min(MAX_SCALE, Math.max(MIN_SCALE, state.scale + delta * state.scale))
-    state.scale = next
-    if (next === MIN_SCALE) { state.x = 0; state.y = 0 }
+    state.smooth = false
+    if (ev.ctrlKey) {
+      const [px, py] = fromCentre(ev)
+      zoomAt(state.scale * Math.exp(-ev.deltaY * PINCH_SENSITIVITY), px, py)
+      return
+    }
+    if (state.scale === MIN_SCALE) return
+    const unit = ev.deltaMode === 1 ? LINE_PX
+      : ev.deltaMode === 2 ? (geometry()?.ch || 800) : 1
+    let dx = ev.deltaX * unit
+    let dy = ev.deltaY * unit
+    if (ev.shiftKey && !dx) { dx = dy; dy = 0 }
+    state.x -= dx
+    state.y -= dy
+    clamp()
   }
 
   function onPointerDown(ev) {
-    if (state.scale === 1) return
+    if (ev.button !== 0) return
+    dragged = false
+    press = { x: ev.clientX, y: ev.clientY, panX: state.x, panY: state.y, id: ev.pointerId }
+    if (state.scale === MIN_SCALE) return
     state.panning = true
-    dragOriginX = ev.clientX
-    dragOriginY = ev.clientY
-    panOriginX = state.x
-    panOriginY = state.y
-    ev.currentTarget.setPointerCapture(ev.pointerId)
+    state.smooth = false
+    try { ev.currentTarget.setPointerCapture(ev.pointerId) } catch {}
   }
+
   function onPointerMove(ev) {
+    if (!press) return
+    const dx = ev.clientX - press.x
+    const dy = ev.clientY - press.y
+    if (!dragged && Math.hypot(dx, dy) > DRAG_PX) dragged = true
     if (!state.panning) return
-    state.x = panOriginX + (ev.clientX - dragOriginX)
-    state.y = panOriginY + (ev.clientY - dragOriginY)
+    state.x = press.panX + dx
+    state.y = press.panY + dy
+    clamp()
   }
+
   function onPointerUp(ev) {
+    press = null
+    if (!state.panning) return
     state.panning = false
     try { ev.currentTarget.releasePointerCapture(ev.pointerId) } catch {}
   }
 
+  // True once for the click that ends a drag, so the canvas can ignore it.
+  function endedDrag() {
+    const was = dragged
+    dragged = false
+    return was
+  }
+
   return {
     state,
     handlers: {
@@ -74,8 +176,14 @@ export function usePanZoom() {
       onPointerDown,
       onPointerMove,
       onPointerUp,
-      onClick: toggleZoom
+      onPointerCancel: onPointerUp,
+      onClick: toggleZoom,
     },
-    reset
+    endedDrag,
+    zoomBy,
+    setScale,
+    maxScale,
+    clamp,
+    reset,
   }
 }

frontend/test/usePanZoom.spec.js

@@ -0,0 +1,75 @@
+import { describe, it, expect } from 'vitest'
+
+import { usePanZoom } from '../src/composables/usePanZoom.js'
+
+// A 1400x1300 canvas. The strip is 3600x24000, fitted to 195x1300.
+const STRIP = { cw: 1400, ch: 1300, iw: 195, ih: 1300, nw: 3600 }
+const PHOTO = { cw: 1400, ch: 1300, iw: 1200, ih: 900, nw: 4000 }
+
+const RECT = { left: 0, top: 0, width: 1400, height: 1300 }
+const target = { getBoundingClientRect: () => RECT, setPointerCapture() {}, releasePointerCapture() {} }
+const at = (x, y, extra = {}) => ({ clientX: x, clientY: y, currentTarget: target,
+  button: 0, pointerId: 1, preventDefault() {}, ...extra })
+
+describe('usePanZoom', () => {
+  it('opens a long strip at the canvas width, keeping the clicked point under the cursor', () => {
+    const pz = usePanZoom(() => STRIP)
+    pz.handlers.onClick(at(700, 650 - 300))
+    expect(pz.state.scale).toBeCloseTo(1400 / 195)
+    expect(pz.state.x).toBe(0)
+    // The picture point that was 300px above centre (picture y = -300 at
+    // scale 1) is still 300px above centre: y + scale * -300 === -300.
+    expect(pz.state.y + pz.state.scale * -300).toBeCloseTo(-300)
+  })
+
+  it('doubles an ordinary picture', () => {
+    const pz = usePanZoom(() => PHOTO)
+    pz.handlers.onClick(at(700, 650))
+    expect(pz.state.scale).toBe(2)
+  })
+
+  it('does not unzoom on the click that ends a drag', () => {
+    const pz = usePanZoom(() => STRIP)
+    pz.handlers.onClick(at(700, 650))
+    pz.handlers.onPointerDown(at(700, 650))
+    pz.handlers.onPointerMove(at(700, 450))
+    pz.handlers.onPointerUp(at(700, 450))
+    expect(pz.endedDrag()).toBe(true)
+    expect(pz.state.y).toBe(-200)
+    // A plain click afterwards is a click again.
+    pz.handlers.onPointerDown(at(700, 450))
+    pz.handlers.onPointerUp(at(700, 450))
+    expect(pz.endedDrag()).toBe(false)
+  })
+
+  it('scrolls a zoomed picture with the wheel and stops at its ends', () => {
+    const pz = usePanZoom(() => STRIP)
+    pz.handlers.onClick(at(700, 650))
+    const scale = pz.state.scale
+    pz.handlers.onWheel(at(700, 650, { deltaY: 100, deltaX: 0, deltaMode: 0 }))
+    expect(pz.state.scale).toBe(scale)
+    expect(pz.state.y).toBe(-100)
+    pz.handlers.onWheel(at(700, 650, { deltaY: 1e6, deltaX: 0, deltaMode: 0 }))
+    expect(pz.state.y).toBeCloseTo(-(1300 * scale - 1300) / 2)
+  })
+
+  it('leaves the fitted view alone on a plain wheel and zooms with ctrl held', () => {
+    const pz = usePanZoom(() => PHOTO)
+    pz.handlers.onWheel(at(700, 650, { deltaY: 100, deltaX: 0, deltaMode: 0 }))
+    expect(pz.state.scale).toBe(1)
+    pz.handlers.onWheel(at(700, 650, { deltaY: -100, deltaX: 0, deltaMode: 0, ctrlKey: true }))
+    expect(pz.state.scale).toBeGreaterThan(1)
+  })
+
+  it('steps with the buttons and never goes past full size twice over', () => {
+    const pz = usePanZoom(() => PHOTO)
+    pz.zoomBy(1)
+    expect(pz.state.scale).toBeCloseTo(1.25)
+    pz.zoomBy(100)
+    expect(pz.state.scale).toBeCloseTo(pz.maxScale())
+    expect(pz.maxScale()).toBeCloseTo((4000 / 1200) * 2)
+    pz.zoomBy(-100)
+    expect(pz.state.scale).toBe(1)
+    expect([pz.state.x, pz.state.y]).toEqual([0, 0])
+  })
+})