diff --git a/messages_essentials.go b/messages_essentials.go
index 7955b6e..618816a 100644
--- a/messages_essentials.go
+++ b/messages_essentials.go
@@ -28,6 +28,8 @@ func init() {
 
 		"undo_toast.undo": {"Undo", "撤销"},
 
+		"resizable_panel_group.divider": {"Resize %s and %s", "调整 %s 与 %s 的大小"},
+
 		"qr_code.label":    {"QR code", "二维码"},
 		"qr_code.too_long": {"Text too long for a QR code", "文字过长，无法生成二维码"},
 	})
diff --git a/resizable_panel_group.go b/resizable_panel_group.go
new file mode 100644
index 0000000..8e273b6
--- /dev/null
+++ b/resizable_panel_group.go
@@ -0,0 +1,256 @@
+package mujica
+
+import (
+	"cmp"
+	"fmt"
+	"log/slog"
+	"math"
+
+	"github.com/egoist/mygo/ui"
+
+	"mujicaui/internal"
+	"mujicaui/theme"
+)
+
+// ResizablePanel is one panel of a ResizablePanelGroup.
+type ResizablePanel struct {
+	// Label names the panel; dividers read it out.
+	Label string
+	// Min is the smallest size in DIPs; zero is 10 spacing units.
+	Min         float32
+	Collapsible bool
+	Build       func()
+}
+
+// ResizablePanelGroupOptions configure a ResizablePanelGroup.
+type ResizablePanelGroupOptions struct {
+	// Vertical stacks the panels top to bottom.
+	Vertical bool
+	Disabled bool
+}
+
+// ResizablePanelGroupResult reports a group's changes: Resized while moving, Committed when a change is done, to save.
+type ResizablePanelGroupResult struct {
+	Element   *ui.Element
+	Resized   bool
+	Committed bool
+}
+
+// resizablePanelState keeps drags and the sizes collapsed panels return to.
+type resizablePanelState struct {
+	held     int // divider being dragged, -1 for none
+	restored map[int]float32
+}
+
+// ResizablePanelGroup lays panels in a row with dividers; *sizes holds their fractions, summing to 1, for the caller to save.
+func ResizablePanelGroup(c *ui.Context, sizes *[]float32, panels []ResizablePanel, opts ResizablePanelGroupOptions) ResizablePanelGroupResult {
+	if sizes == nil || len(panels) < 2 {
+		panic("mujica: ResizablePanelGroup needs sizes and two panels or more")
+	}
+	n := len(panels)
+	if len(*sizes) == 0 {
+		*sizes = make([]float32, n)
+		for i := range *sizes {
+			(*sizes)[i] = 1 / float32(n)
+		}
+	}
+	if len(*sizes) != n {
+		panic(fmt.Sprintf("mujica: ResizablePanelGroup has %d sizes for %d panels", len(*sizes), n))
+	}
+	var sum float32
+	for i, f := range *sizes {
+		if !(f >= 0) || f == 0 && !panels[i].Collapsible || panels[i].Label == "" || panels[i].Build == nil || panels[i].Min < 0 {
+			panic(fmt.Sprintf("mujica: ResizablePanelGroup panel %d is invalid", i))
+		}
+		sum += f
+	}
+	if math.Abs(float64(sum-1)) > 1e-3 {
+		panic(fmt.Sprintf("mujica: ResizablePanelGroup sizes sum to %v, not 1", sum))
+	}
+	u := Density(c).Unit()
+	k := Tokens(c)
+	var res ResizablePanelGroupResult
+	box := ui.Row(c).AlignItems(ui.Stretch).Disabled(opts.Disabled)
+	if opts.Vertical {
+		box.Column()
+	}
+	disabled := box.IsDisabled()
+	st := *ui.Local(box, "resizable_panel_group", func() *resizablePanelState { return &resizablePanelState{held: -1, restored: map[int]float32{}} })
+	was := box.Bounds()
+	internal.Remeasure(c, box, was.W, was.H)
+	total := was.W
+	if opts.Vertical {
+		total = was.H
+	}
+	avail := max(0, total-float32(n-1)*theme.BorderWidth)
+	mins := make([]float32, n)
+	for i, p := range panels {
+		mins[i] = cmp.Or(p.Min, 10*u)
+	}
+	along := func(e *ui.Element, v float32) *ui.Element {
+		if opts.Vertical {
+			return e.Height(v).FillWidth()
+		}
+		return e.Width(v).FillHeight()
+	}
+	box.Children(func() {
+		var grips []func()
+		defer func() {
+			// Grips go last, above the panels they overlap.
+			for _, g := range grips {
+				g()
+			}
+		}()
+		for i, p := range panels {
+			if i > 0 {
+				grips = append(grips, resizablePanelDivider(c, k, u, st, sizes, panels, mins, avail, i-1, opts.Vertical, disabled, &res))
+			}
+			if (*sizes)[i] == 0 {
+				continue
+			}
+			pane := ui.Box(c).Key(i).Shrink(0).Clip().Role(ui.RoleGroup).Label(p.Label)
+			if avail > 0 {
+				along(pane, (*sizes)[i]*avail)
+			} else {
+				pane.Grow((*sizes)[i])
+			}
+			pane.Children(p.Build)
+		}
+	})
+	if res.Resized || res.Committed {
+		c.Invalidate()
+	}
+	res.Element = box
+	return res
+}
+
+// resizablePanelDivider builds the divider after panel i, stepped with arrows, Enter collapsing; it returns the grip's builder.
+func resizablePanelDivider(c *ui.Context, k theme.Tokens, u float32, st *resizablePanelState, sizes *[]float32, panels []ResizablePanel, mins []float32, avail float32, i int, vertical, disabled bool, res *ResizablePanelGroupResult) func() {
+	s := *sizes
+	div := ui.Box(c).Key(fmt.Sprint("div", i)).Shrink(0).Focusable().FocusRing(false).Role(ui.RoleSplitter).
+		Label(fmt.Sprintf(msg(c, "resizable_panel_group.divider"), panels[i].Label, panels[i+1].Label))
+	if vertical {
+		div.Height(theme.BorderWidth).FillWidth()
+	} else {
+		div.Width(theme.BorderWidth).FillHeight()
+	}
+	// Moving the boundary by d DIPs trades size between panels i and i+1 within their minimums.
+	move := func(d float32) bool {
+		if avail <= 0 {
+			return false
+		}
+		a, b := s[i]*avail, s[i+1]*avail
+		pair := a + b
+		na := max(0, min(pair, a+d))
+		collapse := func(size, min float32, ok bool) bool { return ok && size < min/2 }
+		switch {
+		case collapse(na, mins[i], panels[i].Collapsible):
+			na = 0
+		case collapse(pair-na, mins[i+1], panels[i+1].Collapsible):
+			na = pair
+		default:
+			na = max(min(mins[i], pair-mins[i+1]), min(na, pair-mins[i+1]))
+		}
+		if math.Abs(float64(na-a)) < 0.01 {
+			return false
+		}
+		s[i], s[i+1] = na/avail, (pair-na)/avail
+		return true
+	}
+	toggle := func() {
+		j := i + 1
+		if !panels[j].Collapsible {
+			j = i
+		}
+		if !panels[j].Collapsible {
+			return
+		}
+		other := i + i + 1 - j
+		if s[j] > 0 {
+			st.restored[j] = s[j]
+			s[other] += s[j]
+			s[j] = 0
+		} else {
+			back := min(cmp.Or(st.restored[j], mins[j]/max(avail, 1)), s[other]-mins[other]/max(avail, 1))
+			s[j], s[other] = back, s[other]-back
+		}
+		res.Committed = true
+		slog.Debug("mujica: resizable_panel_group toggled", "panel", panels[j].Label, "size", s[j])
+	}
+	if !disabled {
+		back, forth := ui.KeyLeft, ui.KeyRight
+		if vertical {
+			back, forth = ui.KeyUp, ui.KeyDown
+		}
+		switch {
+		case div.Shortcut(0, back):
+			res.Committed = move(-4 * u)
+		case div.Shortcut(0, forth):
+			res.Committed = move(4 * u)
+		case div.Shortcut(0, ui.KeyHome):
+			res.Committed = move(-s[i] * avail)
+		case div.Shortcut(0, ui.KeyEnd):
+			res.Committed = move(s[i+1] * avail)
+		case div.Shortcut(0, ui.KeyEnter):
+			toggle()
+		}
+	}
+	if s[i] > 0 && s[i+1] > 0 {
+		div.Description(fmt.Sprintf("%.0f%%", 100*s[i]/(s[i]+s[i+1])))
+	} else {
+		div.Description(msg(c, "split_pane.collapsed"))
+	}
+	return func() {
+		grip := ui.Box(c).Key(fmt.Sprint("grip", i)).Absolute().Cursor(ui.CursorResizeEW)
+		if vertical {
+			grip.Cursor(ui.CursorResizeNS)
+		}
+		dx, dy, held := grip.Dragged()
+		held = held && !disabled
+		switch {
+		case held:
+			if st.held != i {
+				st.held = i
+				div.Focus()
+			}
+			d := dx
+			if vertical {
+				d = dy
+			}
+			if d != 0 && move(d) {
+				res.Resized = true
+			}
+		case st.held == i:
+			st.held = -1
+			res.Committed = true
+			slog.Debug("mujica: resizable_panel_group resized", "divider", i, "sizes", s)
+		}
+		resizablePanelGrip(c, k, u, s, avail, i, vertical, grip, div, st.held == i)
+	}
+}
+
+// resizablePanelGrip places the grip over divider i and draws the divider.
+func resizablePanelGrip(c *ui.Context, k theme.Tokens, u float32, s []float32, avail float32, i int, vertical bool, grip, div *ui.Element, held bool) {
+	// The grip is wider than the 1 DIP line, centered on the boundary.
+	var at float32
+	for _, f := range s[:i+1] {
+		at += f * avail
+	}
+	at += float32(i) * theme.BorderWidth
+	if vertical {
+		grip.Left(0).Right(0).Top(at - 1.5*u).Height(3 * u)
+	} else {
+		grip.Top(0).Bottom(0).Left(at - 1.5*u).Width(3 * u)
+	}
+	active := held || grip.Hovered()
+	div.Draw(func(p *ui.Painter, r ui.Rect) {
+		line := k.ControlBorder
+		if active {
+			line = k.AccentText
+		}
+		p.Fill(r, line, 0)
+		if div.FocusVisible() {
+			internal.FocusRing(p, r, 0, k.Focus)
+		}
+	})
+}
diff --git a/resizable_panel_group_test.go b/resizable_panel_group_test.go
new file mode 100644
index 0000000..7deb347
--- /dev/null
+++ b/resizable_panel_group_test.go
@@ -0,0 +1,135 @@
+package mujica
+
+import (
+	"math"
+	"testing"
+
+	"github.com/egoist/mygo/ui"
+)
+
+func panelTester(sizes *[]float32, opts *ResizablePanelGroupOptions, commits *int) *ui.Tester {
+	panels := []ResizablePanel{
+		{Label: "Files", Min: 100, Collapsible: true, Build: func() {}},
+		{Label: "Editor", Min: 150, Build: func() {}},
+		{Label: "Outline", Min: 80, Build: func() {}},
+	}
+	return ui.NewTester(func(c *ui.Context) {
+		Use(c, Settings{})
+		r := ResizablePanelGroup(c, sizes, panels, *opts)
+		r.Element.Size(602, 300)
+		if r.Committed {
+			*commits++
+		}
+	}, 700, 400)
+}
+
+func near(a, b float32) bool { return math.Abs(float64(a-b)) < 0.002 }
+
+func TestResizablePanelGroupKeyboard(t *testing.T) {
+	var sizes []float32
+	opts := ResizablePanelGroupOptions{}
+	commits := 0
+	tt := panelTester(&sizes, &opts, &commits)
+	tt.Frame()
+	if len(sizes) != 3 || !near(sizes[0], 1.0/3) {
+		t.Fatalf("default sizes %v", sizes)
+	}
+	tt.Key(0, ui.KeyTab)
+	if !tt.Focused("Resize Files and Editor") {
+		t.Fatal("first divider not focused")
+	}
+	tt.Key(0, ui.KeyRight) // 16 DIPs of 600
+	if !near(sizes[0], (200.0+16)/600) || !near(sizes[1], (200.0-16)/600) || commits != 1 {
+		t.Fatalf("Right: %v (%d)", sizes, commits)
+	}
+	tt.Key(0, ui.KeyEnd) // Editor keeps its 150 minimum
+	if !near(sizes[1], 150.0/600) {
+		t.Fatalf("End: %v", sizes)
+	}
+	tt.Key(0, ui.KeyEnter)
+	if sizes[0] != 0 || !near(sizes[1], 1-sizes[2]) {
+		t.Fatalf("collapse: %v", sizes)
+	}
+	tt.Key(0, ui.KeyEnter)
+	if !near(sizes[0], 250.0/600) {
+		t.Fatalf("restore: %v", sizes)
+	}
+	opts.Disabled = true
+	tt.Frame()
+	before := sizes[0]
+	tt.Key(0, ui.KeyLeft)
+	if sizes[0] != before {
+		t.Fatal("disabled group resized")
+	}
+}
+
+func TestResizablePanelGroupDrag(t *testing.T) {
+	sizes := []float32{0.5, 0.25, 0.25}
+	opts := ResizablePanelGroupOptions{}
+	commits := 0
+	tt := panelTester(&sizes, &opts, &commits)
+	tt.Frame()
+	x := float32(300) // the first boundary
+	tt.Press(x, 100)
+	tt.Move(x-60, 100)
+	if commits != 0 || !near(sizes[0], 240.0/600) {
+		t.Fatalf("while dragging: %v (%d)", sizes, commits)
+	}
+	tt.Release(x-60, 100)
+	if commits != 1 {
+		t.Fatalf("release did not commit: %d", commits)
+	}
+	tt.Press(x-60, 100)
+	tt.Move(x-60-200, 100) // past half of Files' minimum: it collapses
+	tt.Release(x-60-200, 100)
+	if sizes[0] != 0 {
+		t.Fatalf("drag collapse: %v", sizes)
+	}
+}
+
+func TestResizablePanelGroupRejects(t *testing.T) {
+	ok := ResizablePanel{Label: "A", Build: func() {}}
+	for name, tc := range map[string]struct {
+		sizes  []float32
+		panels []ResizablePanel
+	}{
+		"one panel":  {nil, []ResizablePanel{ok}},
+		"count":      {[]float32{1}, []ResizablePanel{ok, ok}},
+		"sum":        {[]float32{0.5, 0.2}, []ResizablePanel{ok, ok}},
+		"zero fixed": {[]float32{0, 1}, []ResizablePanel{ok, ok}},
+		"no label":   {nil, []ResizablePanel{ok, {Build: func() {}}}},
+	} {
+		func() {
+			defer func() {
+				if recover() == nil {
+					t.Errorf("%s: no panic", name)
+				}
+			}()
+			s := tc.sizes
+			ui.NewTester(func(c *ui.Context) { Use(c, Settings{}); ResizablePanelGroup(c, &s, tc.panels, ResizablePanelGroupOptions{}) }, 300, 200)
+		}()
+	}
+}
+
+func TestResizablePanelGroupSnapshot(t *testing.T) {
+	sizes := []float32{0.25, 0.5, 0.25}
+	vs := []float32{0.6, 0.4}
+	snapshot(t, "resizable_panel_group", 560, 260, func(c *ui.Context) {
+		k := Tokens(c)
+		pane := func(s string) func() {
+			return func() {
+				ui.Column(c).Fill().Padding(12).Background(k.Surface).Children(func() { ui.Text(c, s) })
+			}
+		}
+		ui.Row(c).Padding(16).Children(func() {
+			ResizablePanelGroup(c, &sizes, []ResizablePanel{
+				{Label: "Files", Build: pane("Files")},
+				{Label: "Editor", Build: func() {
+					ResizablePanelGroup(c, &vs, []ResizablePanel{{Label: "Code", Build: pane("Code")}, {Label: "Terminal", Build: pane("Terminal")}},
+						ResizablePanelGroupOptions{Vertical: true}).Element.Fill()
+				}},
+				{Label: "Outline", Build: pane("Outline")},
+			}, ResizablePanelGroupOptions{}).Element.Size(528, 228)
+		})
+	})
+}
diff --git a/testdata/snapshots/darwin/resizable_panel_group-dark.png b/testdata/snapshots/darwin/resizable_panel_group-dark.png
new file mode 100644
index 0000000..a7a9b63
Binary files /dev/null and b/testdata/snapshots/darwin/resizable_panel_group-dark.png differ
diff --git a/testdata/snapshots/darwin/resizable_panel_group-light.png b/testdata/snapshots/darwin/resizable_panel_group-light.png
new file mode 100644
index 0000000..70099f4
Binary files /dev/null and b/testdata/snapshots/darwin/resizable_panel_group-light.png differ