diff --git a/masonry.go b/masonry.go
new file mode 100644
index 0000000..3c1d7ad
--- /dev/null
+++ b/masonry.go
@@ -0,0 +1,139 @@
+package mujica
+
+import (
+	"cmp"
+	"log/slog"
+	"math"
+	"sort"
+
+	"github.com/egoist/mygo/ui"
+)
+
+// MasonryOptions configure a Masonry.
+type MasonryOptions struct {
+	// Key is required: a stable, unique key per item.
+	Key func(i int) string
+	// ColumnWidth is the narrowest column; zero is 220. Columns fill the width.
+	ColumnWidth float32
+	// Gap is between items; zero is 3 spacing units.
+	Gap float32
+	// Estimate is the height assumed until an item is measured; zero is 160.
+	Estimate float32
+}
+
+// masonryState is the scroll, the measured heights and the cached placement.
+type masonryState struct {
+	scroll  ui.ScrollState
+	heights map[string]float32
+	version int
+	laidOut masonryLayout
+	// built are the items built last frame; their keys are checked for changed data.
+	built []int
+	// sized: a frame was asked for once the box had a width.
+	sized bool
+}
+
+// masonryLayout places n items in cols columns; byCol lists each column's items top to bottom.
+type masonryLayout struct {
+	n, cols, version int
+	colW, gap        float32
+	keys             []string
+	x, y, h          []float32
+	byCol            [][]int
+	total            float32
+}
+
+// Masonry lays n items in as many columns as fit, each into the shortest column, building only those in view.
+func Masonry(c *ui.Context, n int, opts MasonryOptions, item func(i int)) *ui.Element {
+	if opts.Key == nil || item == nil {
+		panic("mujica: Masonry needs Key and item")
+	}
+	if n < 0 || opts.ColumnWidth < 0 || opts.Gap < 0 || opts.Estimate < 0 {
+		panic("mujica: Masonry count and sizes must not be negative")
+	}
+	u := Density(c).Unit()
+	colMin := cmp.Or(opts.ColumnWidth, 220)
+	gap := cmp.Or(opts.Gap, 3*u)
+	est := cmp.Or(opts.Estimate, 160)
+	sc := ui.Scroll(c)
+	st := ui.Local(sc, "masonry", func() *masonryState { return &masonryState{heights: map[string]float32{}} })
+	s := *st
+	sc.TrackScroll(&s.scroll)
+	view := sc.Bounds()
+	if view.W == 0 && !s.sized {
+		// Unmeasured: lay out again once the box has its width.
+		s.sized = true
+		c.Invalidate()
+	}
+	cols := max(1, int((view.W+gap)/(colMin+gap)))
+	colW := (view.W - gap*float32(cols-1)) / float32(cols)
+	l := &s.laidOut
+	stale := l.n != n || l.cols != cols || l.colW != colW || l.gap != gap || l.version != s.version
+	for _, i := range s.built {
+		stale = stale || i >= n || l.keys[i] != opts.Key(i)
+	}
+	if stale {
+		*l = masonryPlace(n, cols, colW, gap, est, s.heights, opts.Key)
+		l.version = s.version
+		slog.Debug("mujica: masonry layout", "items", n, "columns", cols)
+	}
+	top, bottom := s.scroll.Y-view.H/2, s.scroll.Y+view.H*1.5
+	var seen []int
+	sc.Children(func() {
+		ui.Box(c).FillWidth().Height(l.total).Children(func() { masonryBuild(c, s, l, colW, top, bottom, item, &seen) })
+	})
+	s.built = seen
+	return sc
+}
+
+// masonryBuild builds the items between top and bottom and records measured heights.
+func masonryBuild(c *ui.Context, s *masonryState, l *masonryLayout, colW, top, bottom float32, item func(int), seen *[]int) {
+	for _, list := range l.byCol {
+		from := sort.Search(len(list), func(j int) bool { i := list[j]; return l.y[i]+l.h[i] >= top })
+		for _, i := range list[from:] {
+			if l.y[i] > bottom {
+				break
+			}
+			*seen = append(*seen, i)
+			key := l.keys[i]
+			box := ui.Column(c).Key(key).Absolute().Left(l.x[i]).Top(l.y[i]).Width(colW)
+			box.Children(func() { item(i) })
+			if h := box.Bounds().H; h > 0 && math.Abs(float64(h-l.h[i])) > 0.5 {
+				s.heights[key] = h
+				s.version++
+				c.Invalidate()
+			}
+		}
+	}
+}
+
+// masonryPlace puts each item into the shortest column, using measured heights or est.
+func masonryPlace(n, cols int, colW, gap, est float32, heights map[string]float32, key func(int) string) masonryLayout {
+	l := masonryLayout{n: n, cols: cols, colW: colW, gap: gap, keys: make([]string, n), x: make([]float32, n), y: make([]float32, n), h: make([]float32, n), byCol: make([][]int, cols)}
+	colH := make([]float32, cols)
+	seen := make(map[string]bool, n)
+	for i := range n {
+		k := key(i)
+		if seen[k] {
+			panic("mujica: Masonry key repeated: " + k)
+		}
+		seen[k] = true
+		col := 0
+		for j := range cols {
+			if colH[j] < colH[col] {
+				col = j
+			}
+		}
+		h, ok := heights[k]
+		if !ok {
+			h = est
+		}
+		l.keys[i], l.x[i], l.y[i], l.h[i] = k, float32(col)*(colW+gap), colH[col], h
+		l.byCol[col] = append(l.byCol[col], i)
+		colH[col] += h + gap
+	}
+	for _, h := range colH {
+		l.total = max(l.total, h-gap)
+	}
+	return l
+}
diff --git a/masonry_test.go b/masonry_test.go
new file mode 100644
index 0000000..71349e8
--- /dev/null
+++ b/masonry_test.go
@@ -0,0 +1,78 @@
+package mujica
+
+import (
+	"strconv"
+	"testing"
+
+	"github.com/egoist/mygo/ui"
+)
+
+func TestMasonryPlaceShortestColumn(t *testing.T) {
+	hs := map[string]float32{"0": 100, "1": 40, "2": 40, "3": 50}
+	l := masonryPlace(4, 2, 100, 10, 160, hs, strconv.Itoa)
+	// 0 left; 1 right; 2 right again (50 < 110); 3 right (100 = 110? no: left 110, right 100).
+	want := [][2]float32{{0, 0}, {110, 0}, {110, 50}, {110, 100}}
+	for i, w := range want {
+		if l.x[i] != w[0] || l.y[i] != w[1] {
+			t.Errorf("item %d at %v,%v, want %v", i, l.x[i], l.y[i], w)
+		}
+	}
+	if l.total != 150 {
+		t.Errorf("total %v", l.total)
+	}
+	defer func() {
+		if recover() == nil {
+			t.Error("repeated key accepted")
+		}
+	}()
+	masonryPlace(2, 1, 100, 0, 10, nil, func(int) string { return "same" })
+}
+
+func TestMasonryVirtualizesAndMeasures(t *testing.T) {
+	built := map[int]bool{}
+	n := 100000
+	width := 700
+	tt := ui.NewTester(func(c *ui.Context) {
+		Use(c, Settings{})
+		Masonry(c, n, MasonryOptions{Key: strconv.Itoa, ColumnWidth: 200, Gap: 10}, func(i int) {
+			built[i] = true
+			ui.Box(c).Height(float32(40 + i%3*30)).Label("tile " + strconv.Itoa(i))
+		}).Size(float32(width), 500)
+	}, 800, 600)
+	tt.Frame()
+	clear(built)
+	tt.Frame()
+	if len(built) == 0 || len(built) > 120 {
+		t.Fatalf("built %d of %d", len(built), n)
+	}
+	if !built[0] || !built[2] || built[5000] {
+		t.Fatalf("wrong items built: %v", built[5000])
+	}
+	a, _ := tt.Find("tile 0")
+	b, _ := tt.Find("tile 1")
+	d, _ := tt.Find("tile 3")
+	if a.W < 226 || a.W > 227 || b.X < 236 || b.X > 237 {
+		t.Fatalf("three columns in 700: tile 0 %v, tile 1 %v", a, b)
+	}
+	// Measured: tile 0 is 40 high, so tile 3 goes under it at 40 + 10.
+	if d.X != 0 || d.Y != 50 {
+		t.Fatalf("tile 3 at %v; texts %q", d, tt.Texts())
+	}
+	tt.Scroll(100, 100, 0, 600000)
+	clear(built)
+	tt.Frame()
+	if len(built) == 0 || len(built) > 120 || built[0] {
+		t.Fatalf("after scrolling built %d, includes first %v", len(built), built[0])
+	}
+}
+
+func TestMasonrySnapshot(t *testing.T) {
+	snapshot(t, "masonry", 520, 300, func(c *ui.Context) {
+		k := Tokens(c)
+		Masonry(c, 9, MasonryOptions{Key: strconv.Itoa, ColumnWidth: 150}, func(i int) {
+			ui.Column(c).Height(float32(50+i%4*25)).Padding(8).Background(k.Surface).Border(1, k.Border).Radius(4).Children(func() {
+				ui.Text(c, "Card "+strconv.Itoa(i+1))
+			})
+		}).Size(500, 280).Margin(10)
+	})
+}
diff --git a/testdata/snapshots/darwin/masonry-dark.png b/testdata/snapshots/darwin/masonry-dark.png
new file mode 100644
index 0000000..4c4ee52
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diff --git a/testdata/snapshots/darwin/masonry-light.png b/testdata/snapshots/darwin/masonry-light.png
new file mode 100644
index 0000000..44e6eb2
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