feat(fc2c-i): usePolyMasonry shortest-column distribution

frontend/src/composables/usePolyMasonry.js

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+// Shortest-column masonry distribution (ported from ImageRepo).
+//
+// Predictive relative height = height / width. Using the aspect ratio means
+// columns are balanced before thumbnails load, so there is no reflow when
+// images arrive. Missing or zero dimensions fall back to a square (1.0).
+
+import { ref, onMounted, onUnmounted } from 'vue'
+
+function relativeHeight(item) {
+  const w = Number(item.width)
+  const h = Number(item.height)
+  if (!w || !h) return 1
+  return h / w
+}
+
+export function distributeIntoColumns(items, columnCount) {
+  const n = Math.max(1, columnCount | 0)
+  const columns = Array.from({ length: n }, () => [])
+  const heights = new Array(n).fill(0)
+  for (const item of items) {
+    let shortest = 0
+    for (let c = 1; c < n; c++) {
+      if (heights[c] < heights[shortest]) shortest = c
+    }
+    columns[shortest].push(item)
+    heights[shortest] += relativeHeight(item)
+  }
+  return columns
+}
+
+// Reactive column count from container width. Breakpoints mirror the
+// gallery grid intent: ~260px target column width, min 1 column.
+export function columnCountForWidth(width) {
+  if (!width || width < 0) return 1
+  return Math.max(1, Math.floor(width / 260))
+}
+
+export function usePolyMasonry(containerRef) {
+  const columnCount = ref(1)
+  let ro = null
+
+  function measure() {
+    const el = containerRef.value
+    if (el) columnCount.value = columnCountForWidth(el.clientWidth)
+  }
+
+  onMounted(() => {
+    measure()
+    if (typeof ResizeObserver !== 'undefined' && containerRef.value) {
+      ro = new ResizeObserver(measure)
+      ro.observe(containerRef.value)
+    }
+  })
+  onUnmounted(() => ro && ro.disconnect())
+
+  return { columnCount, distribute: distributeIntoColumns }
+}

frontend/test/usePolyMasonry.spec.js

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+import { describe, it, expect } from 'vitest'
+import { distributeIntoColumns } from '../src/composables/usePolyMasonry.js'
+
+describe('distributeIntoColumns', () => {
+  it('returns the requested number of columns', () => {
+    const cols = distributeIntoColumns([], 3)
+    expect(cols.length).toBe(3)
+    expect(cols.every(c => c.length === 0)).toBe(true)
+  })
+
+  it('places each item in the then-shortest column', () => {
+    // 3 square items (relative height 1 each), 3 columns -> one per column.
+    const items = [
+      { id: 1, width: 100, height: 100 },
+      { id: 2, width: 100, height: 100 },
+      { id: 3, width: 100, height: 100 }
+    ]
+    const cols = distributeIntoColumns(items, 3)
+    expect(cols.map(c => c.map(i => i.id))).toEqual([[1], [2], [3]])
+  })
+
+  it('routes a 4th item to a balanced column', () => {
+    const items = [
+      { id: 1, width: 100, height: 300 }, // tall -> col0 height 3
+      { id: 2, width: 100, height: 100 }, // col1 height 1
+      { id: 3, width: 100, height: 100 }, // col2 height 1
+      { id: 4, width: 100, height: 100 }  // shortest is col1 (1) -> col1
+    ]
+    const cols = distributeIntoColumns(items, 3)
+    expect(cols[0].map(i => i.id)).toEqual([1])
+    expect(cols[1].map(i => i.id)).toEqual([2, 4])
+    expect(cols[2].map(i => i.id)).toEqual([3])
+  })
+
+  it('treats missing/zero dimensions as a square (height 1)', () => {
+    const cols = distributeIntoColumns([{ id: 9 }], 1)
+    expect(cols[0]).toEqual([{ id: 9 }])
+  })
+})