diff --git a/internal/treemap/nav.go b/internal/treemap/nav.go
new file mode 100644
index 0000000..e947b84
--- /dev/null
+++ b/internal/treemap/nav.go
@@ -0,0 +1,106 @@
+package treemap
+
+// Direction is an arrow-key direction.
+type Direction int
+
+const (
+	Left Direction = iota
+	Right
+	Up
+	Down
+)
+
+// Neighbour finds the block a user expects to reach by pressing an arrow
+// key from the block with ID sel:
+//
+//  1. Only blocks entirely beyond the selected block's edge in the
+//     direction of travel are candidates.
+//  2. Blocks whose span overlaps the selected block's orthogonal span are
+//     strongly preferred over all others.
+//  3. Then the smallest gap along the direction of travel wins.
+//  4. Then the largest overlap with the selected block's span.
+//  5. Then the block whose orthogonal span is closest to the selected
+//     block's centre line (0 if it contains it).
+//  6. Ties break by position (top, then left) and finally ID.
+//
+// It returns false if there is no block in that direction.
+func Neighbour(blocks []Block, sel int64, d Direction) (int64, bool) {
+	var s Block
+	found := false
+	for _, b := range blocks {
+		if b.ID == sel {
+			s, found = b, true
+			break
+		}
+	}
+	if !found {
+		return 0, false
+	}
+	type score struct {
+		tier, gap, ovl, off, y, x int
+		id                        int64
+	}
+	less := func(a, b score) bool {
+		switch {
+		case a.tier != b.tier:
+			return a.tier < b.tier
+		case a.gap != b.gap:
+			return a.gap < b.gap
+		case a.ovl != b.ovl:
+			return a.ovl > b.ovl
+		case a.off != b.off:
+			return a.off < b.off
+		case a.y != b.y:
+			return a.y < b.y
+		case a.x != b.x:
+			return a.x < b.x
+		}
+		return a.id < b.id
+	}
+	var best score
+	have := false
+	for _, c := range blocks {
+		if c.ID == sel {
+			continue
+		}
+		var gap, lo, hi, clo, chi int
+		var ok bool
+		switch d {
+		case Right:
+			ok, gap = c.X >= s.X+s.W, c.X-(s.X+s.W)
+			lo, hi, clo, chi = s.Y, s.Y+s.H, c.Y, c.Y+c.H
+		case Left:
+			ok, gap = c.X+c.W <= s.X, s.X-(c.X+c.W)
+			lo, hi, clo, chi = s.Y, s.Y+s.H, c.Y, c.Y+c.H
+		case Down:
+			ok, gap = c.Y >= s.Y+s.H, c.Y-(s.Y+s.H)
+			lo, hi, clo, chi = s.X, s.X+s.W, c.X, c.X+c.W
+		case Up:
+			ok, gap = c.Y+c.H <= s.Y, s.Y-(c.Y+c.H)
+			lo, hi, clo, chi = s.X, s.X+s.W, c.X, c.X+c.W
+		}
+		if !ok {
+			continue
+		}
+		tier, ovl := 1, min(hi, chi)-max(lo, clo)
+		if ovl > 0 {
+			tier = 0
+		} else {
+			ovl = 0
+		}
+		// Distance (doubled, to stay integral) from the selected centre
+		// line to the candidate's span; 0 if the span contains it.
+		centre := lo + hi
+		off := 0
+		if centre < 2*clo {
+			off = 2*clo - centre
+		} else if centre > 2*chi {
+			off = centre - 2*chi
+		}
+		sc := score{tier, gap, ovl, off, c.Y, c.X, c.ID}
+		if !have || less(sc, best) {
+			best, have = sc, true
+		}
+	}
+	return best.id, have
+}
diff --git a/internal/treemap/treemap.go b/internal/treemap/treemap.go
new file mode 100644
index 0000000..1dd9aab
--- /dev/null
+++ b/internal/treemap/treemap.go
@@ -0,0 +1,189 @@
+// Package treemap is pure geometry: squarified layout on a cell grid,
+// level-of-detail selection and spatial neighbour search. It has no
+// knowledge of filesystems or terminals.
+package treemap
+
+import (
+	"math"
+	"sort"
+)
+
+// Rect is an integer cell rectangle.
+type Rect struct{ X, Y, W, H int }
+
+// Empty reports whether r has no cells.
+func (r Rect) Empty() bool { return r.W <= 0 || r.H <= 0 }
+
+// Area returns W*H.
+func (r Rect) Area() int { return max(r.W, 0) * max(r.H, 0) }
+
+// Contains reports whether the cell (x, y) is inside r.
+func (r Rect) Contains(x, y int) bool { return x >= r.X && x < r.X+r.W && y >= r.Y && y < r.Y+r.H }
+
+// Inset shrinks r by n cells on each side.
+func (r Rect) Inset(n int) Rect {
+	r = Rect{r.X + n, r.Y + n, r.W - 2*n, r.H - 2*n}
+	if r.W < 0 {
+		r.W = 0
+	}
+	if r.H < 0 {
+		r.H = 0
+	}
+	return r
+}
+
+// Item is something to lay out. Size must be > 0 to receive space.
+type Item struct {
+	ID   int64
+	Size float64
+}
+
+// Block is a laid-out item.
+type Block struct {
+	ID int64
+	Rect
+}
+
+// Squarify lays items out inside bounds using the squarified treemap
+// algorithm (Bruls, Huizing, van Wijk). Items with non-positive size are
+// ignored. The output partitions bounds exactly: rectangle edges are snapped
+// to the grid by rounding shared float edges, so there are no gaps and no
+// overlaps. Items that round to zero width or height are omitted. The result
+// is deterministic for a given input set regardless of input order.
+func Squarify(items []Item, bounds Rect) []Block {
+	if bounds.Empty() {
+		return nil
+	}
+	its := make([]Item, 0, len(items))
+	total := 0.0
+	for _, it := range items {
+		if it.Size > 0 && !math.IsInf(it.Size, 0) && !math.IsNaN(it.Size) {
+			its = append(its, it)
+			total += it.Size
+		}
+	}
+	if len(its) == 0 || total <= 0 {
+		return nil
+	}
+	sort.SliceStable(its, func(i, j int) bool {
+		if its[i].Size != its[j].Size {
+			return its[i].Size > its[j].Size
+		}
+		return its[i].ID < its[j].ID
+	})
+	scale := float64(bounds.W) * float64(bounds.H) / total
+	areas := make([]float64, len(its))
+	for i, it := range its {
+		areas[i] = it.Size * scale
+	}
+	frs := layout(areas, float64(bounds.X), float64(bounds.Y), float64(bounds.W), float64(bounds.H))
+	out := make([]Block, 0, len(its))
+	for i, f := range frs {
+		x0, y0 := int(math.Round(f[0])), int(math.Round(f[1]))
+		x1, y1 := int(math.Round(f[0]+f[2])), int(math.Round(f[1]+f[3]))
+		x1 = min(x1, bounds.X+bounds.W)
+		y1 = min(y1, bounds.Y+bounds.H)
+		if x1 > x0 && y1 > y0 {
+			out = append(out, Block{ID: its[i].ID, Rect: Rect{x0, y0, x1 - x0, y1 - y0}})
+		}
+	}
+	return out
+}
+
+// worst returns the worst aspect ratio of a row with the given sum, largest
+// and smallest areas laid along a side of length side.
+func worst(sum, largest, smallest, side float64) float64 {
+	s2, w2 := sum*sum, side*side
+	return math.Max(w2*largest/s2, s2/(w2*smallest))
+}
+
+// layout returns float rectangles [x, y, w, h] for areas sorted descending.
+func layout(areas []float64, x, y, w, h float64) [][4]float64 {
+	out := make([][4]float64, len(areas))
+	n := len(areas)
+	for i := 0; i < n; {
+		short := math.Min(w, h)
+		j := i + 1
+		sum := areas[i]
+		cur := worst(sum, areas[i], areas[i], short)
+		for j < n {
+			s2 := sum + areas[j]
+			if nw := worst(s2, areas[i], areas[j], short); nw > cur {
+				break
+			} else {
+				cur = nw
+			}
+			sum = s2
+			j++
+		}
+		last := j == n
+		if w >= h {
+			thick := sum / h
+			if last || thick > w {
+				thick = w
+			}
+			yy := y
+			for k := i; k < j; k++ {
+				hh := areas[k] / sum * h
+				if k == j-1 {
+					hh = y + h - yy
+				}
+				out[k] = [4]float64{x, yy, thick, hh}
+				yy += hh
+			}
+			x += thick
+			w -= thick
+		} else {
+			thick := sum / w
+			if last || thick > h {
+				thick = h
+			}
+			xx := x
+			for k := i; k < j; k++ {
+				ww := areas[k] / sum * w
+				if k == j-1 {
+					ww = x + w - xx
+				}
+				out[k] = [4]float64{xx, y, ww, thick}
+				xx += ww
+			}
+			y += thick
+			h -= thick
+		}
+		i = j
+	}
+	return out
+}
+
+// Visible decides how many leading items of a size-sorted (descending)
+// list deserve their own block when area cells represent total: items are
+// kept while each would get at least minArea cells and fewer than maxItems
+// have been kept. The remainder should be merged into one group block.
+func Visible(sizes []float64, total float64, area, minArea, maxItems int) int {
+	if total <= 0 || area <= 0 {
+		return 0
+	}
+	k := 0
+	for k < len(sizes) && k < maxItems {
+		if sizes[k]/total*float64(area) < float64(minArea) {
+			break
+		}
+		k++
+	}
+	// Grouping a single leftover item gains nothing; show it directly if it
+	// gets at least one cell.
+	if k == len(sizes)-1 && k < maxItems && sizes[k]/total*float64(area) >= 1 {
+		k++
+	}
+	return k
+}
+
+// At returns the block containing the cell (x, y).
+func At(blocks []Block, x, y int) (int64, bool) {
+	for _, b := range blocks {
+		if b.Contains(x, y) {
+			return b.ID, true
+		}
+	}
+	return 0, false
+}
diff --git a/internal/treemap/treemap_test.go b/internal/treemap/treemap_test.go
new file mode 100644
index 0000000..9638f9c
--- /dev/null
+++ b/internal/treemap/treemap_test.go
@@ -0,0 +1,247 @@
+package treemap
+
+import (
+	"math/rand"
+	"testing"
+)
+
+func checkPartition(t *testing.T, blocks []Block, b Rect, exact bool) {
+	t.Helper()
+	grid := make([]int, b.W*b.H)
+	for _, bl := range blocks {
+		if bl.W <= 0 || bl.H <= 0 {
+			t.Fatalf("non-positive block %+v", bl)
+		}
+		if bl.X < b.X || bl.Y < b.Y || bl.X+bl.W > b.X+b.W || bl.Y+bl.H > b.Y+b.H {
+			t.Fatalf("block %+v outside bounds %+v", bl, b)
+		}
+		for y := bl.Y; y < bl.Y+bl.H; y++ {
+			for x := bl.X; x < bl.X+bl.W; x++ {
+				i := (y-b.Y)*b.W + (x - b.X)
+				if grid[i] != 0 {
+					t.Fatalf("overlap at %d,%d", x, y)
+				}
+				grid[i] = 1
+			}
+		}
+	}
+	if exact {
+		for i, v := range grid {
+			if v == 0 {
+				t.Fatalf("gap at %d,%d", b.X+i%b.W, b.Y+i/b.W)
+			}
+		}
+	}
+}
+
+func TestSquarifyBasic(t *testing.T) {
+	b := Rect{2, 3, 60, 20}
+	items := []Item{{1, 6}, {2, 6}, {3, 4}, {4, 3}, {5, 2}, {6, 2}, {7, 1}}
+	blocks := Squarify(items, b)
+	if len(blocks) != len(items) {
+		t.Fatalf("got %d blocks", len(blocks))
+	}
+	checkPartition(t, blocks, b, true)
+	// Areas roughly proportional.
+	for _, bl := range blocks {
+		want := float64(b.Area()) * items[bl.ID-1].Size / 24
+		if got := float64(bl.Area()); got < want*0.6-4 || got > want*1.4+4 {
+			t.Errorf("block %d area %v want ~%v", bl.ID, got, want)
+		}
+	}
+	// Deterministic regardless of input order.
+	rev := append([]Item(nil), items...)
+	for i, j := 0, len(rev)-1; i < j; i, j = i+1, j-1 {
+		rev[i], rev[j] = rev[j], rev[i]
+	}
+	b2 := Squarify(rev, b)
+	for i := range blocks {
+		if blocks[i] != b2[i] {
+			t.Fatal("layout depends on input order")
+		}
+	}
+}
+
+func TestSquarifyEdgeCases(t *testing.T) {
+	if Squarify(nil, Rect{0, 0, 10, 10}) != nil {
+		t.Fatal("nil items")
+	}
+	if Squarify([]Item{{1, 0}, {2, -5}}, Rect{0, 0, 10, 10}) != nil {
+		t.Fatal("zero sizes should produce nothing")
+	}
+	for _, b := range []Rect{{0, 0, 0, 0}, {0, 0, 1, 1}, {0, 0, 1, 50}, {0, 0, 50, 1}, {0, 0, -3, 4}} {
+		bl := Squarify([]Item{{1, 5}, {2, 3}, {3, 1}}, b)
+		if !b.Empty() {
+			checkPartition(t, bl, b, true)
+		}
+	}
+	// One enormous item and many tiny ones.
+	items := []Item{{0, 1e15}}
+	for i := 1; i < 500; i++ {
+		items = append(items, Item{int64(i), 1})
+	}
+	bl := Squarify(items, Rect{0, 0, 80, 24})
+	checkPartition(t, bl, Rect{0, 0, 80, 24}, true)
+}
+
+func TestResizeRecomputes(t *testing.T) {
+	items := []Item{{1, 50}, {2, 30}, {3, 20}}
+	for _, b := range []Rect{{0, 0, 80, 24}, {0, 0, 120, 40}, {0, 0, 300, 80}, {0, 0, 70, 20}, {0, 0, 180, 50}} {
+		bl := Squarify(items, b)
+		checkPartition(t, bl, b, true)
+		if len(bl) != 3 {
+			t.Fatalf("%v: %d blocks", b, len(bl))
+		}
+	}
+}
+
+func FuzzSquarify(f *testing.F) {
+	f.Add(int64(1), 80, 24, 10)
+	f.Add(int64(2), 3, 2, 100)
+	f.Add(int64(3), 300, 90, 1000)
+	f.Fuzz(func(t *testing.T, seed int64, w, h, n int) {
+		w, h, n = w%400, h%150, n%3000
+		if w < 0 || h < 0 || n < 0 {
+			return
+		}
+		r := rand.New(rand.NewSource(seed))
+		items := make([]Item, n)
+		for i := range items {
+			items[i] = Item{int64(i), float64(r.Int63n(1 << uint(r.Intn(40)+1)))}
+		}
+		b := Rect{r.Intn(10), r.Intn(10), w, h}
+		bl := Squarify(items, b)
+		if !b.Empty() && len(bl) > 0 {
+			checkPartition(t, bl, b, true)
+		}
+	})
+}
+
+func TestVisible(t *testing.T) {
+	sizes := []float64{100, 50, 10, 1, 1, 1}
+	if k := Visible(sizes, 163, 1630, 20, 100); k != 3 {
+		t.Fatalf("k=%d", k)
+	}
+	if k := Visible(sizes, 163, 1630, 20, 2); k != 2 {
+		t.Fatalf("maxItems k=%d", k)
+	}
+	if k := Visible([]float64{10, 1}, 11, 1100, 200, 10); k != 2 {
+		t.Fatalf("single leftover k=%d", k)
+	}
+}
+
+// Layout:
+//
+//	┌───────┬───────┐
+//	│   A   │   B   │
+//	├───┬───┴───────┤
+//	│ C │     D     │
+//	└───┴───────────┘
+func TestNeighbour(t *testing.T) {
+	A := Block{1, Rect{0, 0, 8, 4}}
+	B := Block{2, Rect{8, 0, 8, 4}}
+	C := Block{3, Rect{0, 4, 4, 4}}
+	D := Block{4, Rect{4, 4, 12, 4}}
+	bs := []Block{D, C, B, A} // order must not matter
+	cases := []struct {
+		from int64
+		d    Direction
+		want int64
+		ok   bool
+	}{
+		{1, Right, 2, true}, {2, Left, 1, true}, {1, Down, 3, true}, {2, Down, 4, true},
+		{3, Right, 4, true}, {4, Left, 3, true}, {3, Up, 1, true}, {4, Up, 2, true},
+		{1, Left, 0, false}, {1, Up, 0, false}, {4, Right, 0, false}, {4, Down, 0, false},
+	}
+	for _, c := range cases {
+		got, ok := Neighbour(bs, c.from, c.d)
+		if ok != c.ok || got != c.want {
+			t.Errorf("from %d dir %d: got %d,%v want %d,%v", c.from, c.d, got, ok, c.want, c.ok)
+		}
+	}
+}
+
+func TestNeighbourPrefersAdjacentOverArrayOrder(t *testing.T) {
+	// S is on the left; R is directly right; F is far right and lower but
+	// appears first in the slice.
+	S := Block{1, Rect{0, 0, 10, 5}}
+	R := Block{2, Rect{10, 0, 10, 5}}
+	F := Block{3, Rect{10, 20, 30, 5}}
+	if got, _ := Neighbour([]Block{F, S, R}, 1, Right); got != 2 {
+		t.Fatalf("got %d", got)
+	}
+	// Tall block on the left with three stacked blocks to the right: pick
+	// the one at the centre line.
+	T := Block{10, Rect{0, 0, 10, 9}}
+	r1 := Block{11, Rect{10, 0, 10, 3}}
+	r2 := Block{12, Rect{10, 3, 10, 3}}
+	r3 := Block{13, Rect{10, 6, 10, 3}}
+	if got, _ := Neighbour([]Block{r3, r1, r2, T}, 10, Right); got != 12 {
+		t.Fatalf("centre preference: got %d", got)
+	}
+	if got, _ := Neighbour([]Block{r3, r1, r2, T}, 13, Left); got != 10 {
+		t.Fatalf("back left: got %d", got)
+	}
+}
+
+func TestNeighbourRandomLayoutsAlwaysAdjacent(t *testing.T) {
+	r := rand.New(rand.NewSource(7))
+	for iter := 0; iter < 200; iter++ {
+		items := make([]Item, 2+r.Intn(30))
+		for i := range items {
+			items[i] = Item{int64(i), float64(1 + r.Intn(1000))}
+		}
+		bl := Squarify(items, Rect{0, 0, 120, 40})
+		for _, s := range bl {
+			for _, d := range []Direction{Left, Right, Up, Down} {
+				id, ok := Neighbour(bl, s.ID, d)
+				if !ok {
+					continue
+				}
+				var c Block
+				for _, b := range bl {
+					if b.ID == id {
+						c = b
+					}
+				}
+				// In a partition, if any block touches the edge in direction d
+				// with overlapping span, the chosen one must touch too.
+				var touch bool
+				switch d {
+				case Right:
+					touch = c.X == s.X+s.W
+				case Left:
+					touch = c.X+c.W == s.X
+				case Down:
+					touch = c.Y == s.Y+s.H
+				case Up:
+					touch = c.Y+c.H == s.Y
+				}
+				if !touch {
+					t.Fatalf("iter %d: from %+v dir %d chose non-adjacent %+v", iter, s, d, c)
+				}
+			}
+		}
+	}
+}
+
+func BenchmarkSquarify(b *testing.B) {
+	items := make([]Item, 300)
+	for i := range items {
+		items[i] = Item{int64(i), float64(1 + i*i)}
+	}
+	for b.Loop() {
+		Squarify(items, Rect{0, 0, 200, 60})
+	}
+}
+
+func BenchmarkNeighbour(b *testing.B) {
+	items := make([]Item, 300)
+	for i := range items {
+		items[i] = Item{int64(i), float64(1 + i*i)}
+	}
+	bl := Squarify(items, Rect{0, 0, 200, 60})
+	for b.Loop() {
+		Neighbour(bl, 150, Right)
+	}
+}