diff --git a/ui/layout/doc.go b/ui/layout/doc.go
index f2510cc7..fd9fc888 100644
--- a/ui/layout/doc.go
+++ b/ui/layout/doc.go
@@ -5,28 +5,30 @@ Package layout implements layouts common to GUI programs.
 
 Constraints and dimensions
 
-Constraints and dimensions form the interface between
-layouts and interface child elements. Every layout operation
-start with a set of constraints for acceptable widths and heights
-of a child. The operation ends with the child computing and returning
-its size and baseline (if any).
+Constraints and dimensions form the interface between layouts and
+interface child elements. This package operates on Widgets, functions
+that compute Dimensions from a a set of constraints for acceptable
+widths and heights. Both the constraints and dimensions are maintained
+in an implicit Context to keep the Widget declaration short.
 
 For example, to add space above a widget:
 
-	var cs layout.Constraints = ...
+	ctx := new(layout.Context)
+	ctx.Constraints = ...
 
 	// Configure a top inset.
 	inset := layout.Inset{Top: ui.Dp(8), ...}
 	// Use the inset to lay out a widget.
-	inset.Layout(..., cs, func(cs layout.Constraints) layout.Dimensions {
+	inset.Layout(..., ctx, func() {
 		// Lay out widget and determine its size given the constraints.
-		dimensions := widget.Layout(..., cs)
-		return dimensions
+		...
+		dims := layout.Dimensions{...}
+		ctx.Dimensions = dims
 	})
 
 Note that the example does not generate any garbage even though the
 Inset is transient. Layouts that don't accept user input are designed
-to escape to the heap during their use.
+to not escape to the heap during their use.
 
 Layout operations are recursive: a child in a layout operation can
 itself be another layout. That way, complex user interfaces can
@@ -35,10 +37,10 @@ be created from a few generic layouts.
 This example both aligns and insets a child:
 
 	inset := layout.Inset{...}
-	inset.Layout(..., cs, func(cs layout.Constraints) layout.Dimensions {
+	inset.Layout(..., ctx, func() {
 		align := layout.Align{...}
-		return align.Layout(..., cs, func(cs layout.Constraints) layout.Dimensions {
-			return widget.Layout(..., cs)
+		align.Layout(..., ctx, func() {
+			widget.Layout(..., ctx)
 		})
 	})
 
diff --git a/ui/layout/flex.go b/ui/layout/flex.go
index 6a58b47c..c33dd92a 100644
--- a/ui/layout/flex.go
+++ b/ui/layout/flex.go
@@ -20,9 +20,9 @@ type Flex struct {
 	// Alignment is the alignment in the cross axis.
 	Alignment Alignment
 
+	ctx       *Context
 	macro     ui.MacroOp
 	ops       *ui.Ops
-	cs        Constraints
 	mode      flexMode
 	size      int
 	rigidSize int
@@ -69,13 +69,13 @@ const (
 )
 
 // Init must be called before Rigid or Flexible.
-func (f *Flex) Init(ops *ui.Ops, cs Constraints) *Flex {
+func (f *Flex) Init(ops *ui.Ops, ctx *Context) *Flex {
 	if f.mode > modeBegun {
 		panic("must End the current child before calling Init again")
 	}
 	f.mode = modeBegun
 	f.ops = ops
-	f.cs = cs
+	f.ctx = ctx
 	f.size = 0
 	f.rigidSize = 0
 	f.maxCross = 0
@@ -98,20 +98,23 @@ func (f *Flex) begin(mode flexMode) {
 // from 0 to the remaining space.
 func (f *Flex) Rigid(w Widget) FlexChild {
 	f.begin(modeRigid)
-	mainc := axisMainConstraint(f.Axis, f.cs)
+	cs := f.ctx.Constraints
+	mainc := axisMainConstraint(f.Axis, cs)
 	mainMax := mainc.Max - f.size
 	if mainMax < 0 {
 		mainMax = 0
 	}
-	cs := axisConstraints(f.Axis, Constraint{Max: mainMax}, axisCrossConstraint(f.Axis, f.cs))
-	return f.end(w(cs))
+	cs = axisConstraints(f.Axis, Constraint{Max: mainMax}, axisCrossConstraint(f.Axis, cs))
+	dims := f.ctx.Layout(cs, w)
+	return f.end(dims)
 }
 
 // Flexible is like Rigid, where the main axis size is also constrained to a
 // fraction of the space not taken up by Rigid children.
 func (f *Flex) Flexible(weight float32, w Widget) FlexChild {
 	f.begin(modeFlex)
-	mainc := axisMainConstraint(f.Axis, f.cs)
+	cs := f.ctx.Constraints
+	mainc := axisMainConstraint(f.Axis, cs)
 	var flexSize int
 	if mainc.Max > f.size {
 		flexSize = mainc.Max - f.rigidSize
@@ -125,8 +128,9 @@ func (f *Flex) Flexible(weight float32, w Widget) FlexChild {
 		}
 	}
 	submainc := Constraint{Max: flexSize}
-	cs := axisConstraints(f.Axis, submainc, axisCrossConstraint(f.Axis, f.cs))
-	return f.end(w(cs))
+	cs = axisConstraints(f.Axis, submainc, axisCrossConstraint(f.Axis, cs))
+	dims := f.ctx.Layout(cs, w)
+	return f.end(dims)
 }
 
 // End a child by specifying its dimensions. Pass the returned layout result
@@ -153,9 +157,10 @@ func (f *Flex) end(dims Dimensions) FlexChild {
 
 // Layout a list of children. The order of the children determines their laid
 // out order.
-func (f *Flex) Layout(children ...FlexChild) Dimensions {
-	mainc := axisMainConstraint(f.Axis, f.cs)
-	crossSize := axisCrossConstraint(f.Axis, f.cs).Constrain(f.maxCross)
+func (f *Flex) Layout(children ...FlexChild) {
+	cs := f.ctx.Constraints
+	mainc := axisMainConstraint(f.Axis, cs)
+	crossSize := axisCrossConstraint(f.Axis, cs).Constrain(f.maxCross)
 	var space int
 	if mainc.Min > f.size {
 		space = mainc.Min - f.size
@@ -220,7 +225,7 @@ func (f *Flex) Layout(children ...FlexChild) Dimensions {
 	if baseline == 0 {
 		baseline = sz.Y
 	}
-	return Dimensions{Size: sz, Baseline: baseline}
+	f.ctx.Dimensions = Dimensions{Size: sz, Baseline: baseline}
 }
 
 func axisPoint(a Axis, main, cross int) image.Point {
diff --git a/ui/layout/layout.go b/ui/layout/layout.go
index e1d40993..e6bb85fc 100644
--- a/ui/layout/layout.go
+++ b/ui/layout/layout.go
@@ -37,9 +37,16 @@ type Alignment uint8
 // space.
 type Direction uint8
 
-// Widget is a function that computes a set of dimensions that
-// satisfies the given constraints.
-type Widget func(cs Constraints) Dimensions
+// Widget is a function scope for drawing, processing input and
+// computing dimensions for a user interface element.
+type Widget func()
+
+// Context tracks the current constraints and dimensions during
+// layout.
+type Context struct {
+	Constraints Constraints
+	Dimensions  Dimensions
+}
 
 const (
 	Start Alignment = iota
@@ -65,6 +72,17 @@ const (
 	Vertical
 )
 
+// Layout a widget with a set of constraints and return its
+// dimensions. The previous constraints are restored after layout.
+func (s *Context) Layout(cs Constraints, w Widget) Dimensions {
+	saved := s.Constraints
+	s.Constraints = cs
+	s.Dimensions = Dimensions{}
+	w()
+	s.Constraints = saved
+	return s.Dimensions
+}
+
 // Constrain a value to the range [Min; Max].
 func (c Constraint) Constrain(v int) int {
 	if v < c.Min {
@@ -100,12 +118,12 @@ type Align struct {
 }
 
 // Layout a widget.
-func (in *Inset) Layout(c ui.Config, ops *ui.Ops, cs Constraints, w Widget) Dimensions {
+func (in Inset) Layout(c ui.Config, ops *ui.Ops, ctx *Context, w Widget) {
 	top := c.Px(in.Top)
 	right := c.Px(in.Right)
 	bottom := c.Px(in.Bottom)
 	left := c.Px(in.Left)
-	mcs := cs
+	mcs := ctx.Constraints
 	mcs.Width.Min -= left + right
 	mcs.Width.Max -= left + right
 	if mcs.Width.Min < 0 {
@@ -125,10 +143,10 @@ func (in *Inset) Layout(c ui.Config, ops *ui.Ops, cs Constraints, w Widget) Dime
 	var stack ui.StackOp
 	stack.Push(ops)
 	ui.TransformOp{}.Offset(toPointF(image.Point{X: left, Y: top})).Add(ops)
-	dims := w(mcs)
+	dims := ctx.Layout(mcs, w)
 	stack.Pop()
-	return Dimensions{
-		Size:     cs.Constrain(dims.Size.Add(image.Point{X: right + left, Y: top + bottom})),
+	ctx.Dimensions = Dimensions{
+		Size:     ctx.Constraints.Constrain(dims.Size.Add(image.Point{X: right + left, Y: top + bottom})),
 		Baseline: dims.Baseline + top,
 	}
 }
@@ -140,13 +158,14 @@ func UniformInset(v ui.Value) Inset {
 }
 
 // Layout a widget.
-func (a *Align) Layout(ops *ui.Ops, cs Constraints, w Widget) Dimensions {
+func (a Align) Layout(ops *ui.Ops, st *Context, w Widget) {
 	var macro ui.MacroOp
+	macro.Record(ops)
+	cs := st.Constraints
 	mcs := cs
 	mcs.Width.Min = 0
 	mcs.Height.Min = 0
-	macro.Record(ops)
-	dims := w(mcs)
+	dims := st.Layout(mcs, w)
 	macro.Stop()
 	sz := dims.Size
 	if sz.X < cs.Width.Min {
@@ -173,7 +192,7 @@ func (a *Align) Layout(ops *ui.Ops, cs Constraints, w Widget) Dimensions {
 	ui.TransformOp{}.Offset(toPointF(p)).Add(ops)
 	macro.Add(ops)
 	stack.Pop()
-	return Dimensions{
+	st.Dimensions = Dimensions{
 		Size:     sz,
 		Baseline: dims.Baseline,
 	}
diff --git a/ui/layout/layout_test.go b/ui/layout/layout_test.go
index f4c958a2..839e5d68 100644
--- a/ui/layout/layout_test.go
+++ b/ui/layout/layout_test.go
@@ -19,22 +19,21 @@ var cfg = new(config)
 
 func ExampleInset() {
 	ops := new(ui.Ops)
+	ctx := new(layout.Context)
 
 	// Loose constraints with no minimal size.
-	var cs layout.Constraints
-	cs.Width.Max = 100
-	cs.Height.Max = 100
+	ctx.Constraints.Width.Max = 100
+	ctx.Constraints.Height.Max = 100
 
 	// Inset all edges by 10.
 	inset := layout.UniformInset(ui.Dp(10))
-	dims := inset.Layout(cfg, ops, cs, func(cs layout.Constraints) layout.Dimensions {
+	inset.Layout(cfg, ops, ctx, func() {
 		// Lay out a 50x50 sized widget.
-		dims := layoutWidget(50, 50, cs)
-		fmt.Println(dims.Size)
-		return dims
+		layoutWidget(ctx, 50, 50)
+		fmt.Println(ctx.Dimensions.Size)
 	})
 
-	fmt.Println(dims.Size)
+	fmt.Println(ctx.Dimensions.Size)
 
 	// Output:
 	// (50,50)
@@ -43,19 +42,19 @@ func ExampleInset() {
 
 func ExampleAlign() {
 	ops := new(ui.Ops)
+	ctx := new(layout.Context)
 
 	// Rigid constraints with both minimum and maximum set.
-	cs := layout.RigidConstraints(image.Point{X: 100, Y: 100})
+	ctx.Constraints = layout.RigidConstraints(image.Point{X: 100, Y: 100})
 
 	align := layout.Align{Alignment: layout.Center}
-	dims := align.Layout(ops, cs, func(cs layout.Constraints) layout.Dimensions {
+	align.Layout(ops, ctx, func() {
 		// Lay out a 50x50 sized widget.
-		dims := layoutWidget(50, 50, cs)
-		fmt.Println(dims.Size)
-		return dims
+		layoutWidget(ctx, 50, 50)
+		fmt.Println(ctx.Dimensions.Size)
 	})
 
-	fmt.Println(dims.Size)
+	fmt.Println(ctx.Dimensions.Size)
 
 	// Output:
 	// (50,50)
@@ -64,22 +63,23 @@ func ExampleAlign() {
 
 func ExampleFlex() {
 	ops := new(ui.Ops)
+	ctx := new(layout.Context)
 
-	cs := layout.RigidConstraints(image.Point{X: 100, Y: 100})
+	ctx.Constraints = layout.RigidConstraints(image.Point{X: 100, Y: 100})
 
 	flex := layout.Flex{}
-	flex.Init(ops, cs)
+	flex.Init(ops, ctx)
 
 	// Rigid 10x10 widget.
-	child1 := flex.Rigid(func(cs layout.Constraints) layout.Dimensions {
-		fmt.Printf("Rigid: %v\n", cs.Width)
-		return layoutWidget(10, 10, cs)
+	child1 := flex.Rigid(func() {
+		fmt.Printf("Rigid: %v\n", ctx.Constraints.Width)
+		layoutWidget(ctx, 10, 10)
 	})
 
 	// Child with 50% space allowance.
-	child2 := flex.Flexible(0.5, func(cs layout.Constraints) layout.Dimensions {
-		fmt.Printf("50%%: %v\n", cs.Width)
-		return layoutWidget(10, 10, cs)
+	child2 := flex.Flexible(0.5, func() {
+		fmt.Printf("50%%: %v\n", ctx.Constraints.Width)
+		layoutWidget(ctx, 10, 10)
 	})
 
 	flex.Layout(child1, child2)
@@ -91,21 +91,22 @@ func ExampleFlex() {
 
 func ExampleStack() {
 	ops := new(ui.Ops)
+	ctx := new(layout.Context)
 
-	cs := layout.RigidConstraints(image.Point{X: 100, Y: 100})
+	ctx.Constraints = layout.RigidConstraints(image.Point{X: 100, Y: 100})
 
 	stack := layout.Stack{}
-	stack.Init(ops, cs)
+	stack.Init(ops, ctx)
 
 	// Rigid 50x50 widget.
-	child1 := stack.Rigid(func(cs layout.Constraints) layout.Dimensions {
-		return layoutWidget(50, 50, cs)
+	child1 := stack.Rigid(func() {
+		layoutWidget(ctx, 50, 50)
 	})
 
 	// Force widget to the same size as the first.
-	child2 := stack.Expand(func(cs layout.Constraints) layout.Dimensions {
-		fmt.Printf("Expand: %v\n", cs)
-		return layoutWidget(10, 10, cs)
+	child2 := stack.Expand(func() {
+		fmt.Printf("Expand: %v\n", ctx.Constraints)
+		layoutWidget(ctx, 10, 10)
 	})
 
 	stack.Layout(child1, child2)
@@ -116,17 +117,18 @@ func ExampleStack() {
 
 func ExampleList() {
 	ops := new(ui.Ops)
+	ctx := new(layout.Context)
 
-	cs := layout.RigidConstraints(image.Point{X: 100, Y: 100})
+	ctx.Constraints = layout.RigidConstraints(image.Point{X: 100, Y: 100})
 
 	// The list is 1e6 elements, but only 5 fit the constraints.
 	const listLen = 1e6
 
 	var list layout.List
 	count := 0
-	list.Layout(cfg, q, ops, cs, listLen, func(cs layout.Constraints, i int) layout.Dimensions {
+	list.Layout(cfg, q, ops, ctx, listLen, func(i int) {
 		count++
-		return layoutWidget(20, 20, cs)
+		layoutWidget(ctx, 20, 20)
 	})
 
 	fmt.Println(count)
@@ -135,8 +137,8 @@ func ExampleList() {
 	// 5
 }
 
-func layoutWidget(width, height int, cs layout.Constraints) layout.Dimensions {
-	return layout.Dimensions{
+func layoutWidget(ctx *layout.Context, width, height int) {
+	ctx.Dimensions = layout.Dimensions{
 		Size: image.Point{
 			X: width,
 			Y: height,
diff --git a/ui/layout/list.go b/ui/layout/list.go
index 9e367d9b..6f8f5c0d 100644
--- a/ui/layout/list.go
+++ b/ui/layout/list.go
@@ -58,7 +58,7 @@ type List struct {
 
 // ListElement is a function that computes the dimensions of
 // a list element.
-type ListElement func(cs Constraints, index int) Dimensions
+type ListElement func(index int)
 
 type iterationDir uint8
 
@@ -96,12 +96,16 @@ func (l *List) init(cfg ui.Config, q input.Queue, ops *ui.Ops, cs Constraints, l
 }
 
 // Layout the List and return its dimensions.
-func (l *List) Layout(c ui.Config, q input.Queue, ops *ui.Ops, cs Constraints, len int, w ListElement) Dimensions {
+func (l *List) Layout(c ui.Config, q input.Queue, ops *ui.Ops, ctx *Context, len int, w ListElement) {
+	cs := ctx.Constraints
 	for l.init(c, q, ops, cs, len); l.more(); l.next() {
 		cs := axisConstraints(l.Axis, Constraint{Max: inf}, axisCrossConstraint(l.Axis, l.cs))
-		l.end(w(cs, l.index()))
+		i := l.index()
+		l.end(ctx.Layout(cs, func() {
+			w(i)
+		}))
 	}
-	return l.layout()
+	ctx.Dimensions = l.layout()
 }
 
 func (l *List) scrollToEnd() bool {
diff --git a/ui/layout/stack.go b/ui/layout/stack.go
index 3177c4a8..e4f8accc 100644
--- a/ui/layout/stack.go
+++ b/ui/layout/stack.go
@@ -18,7 +18,7 @@ type Stack struct {
 	macro       ui.MacroOp
 	ops         *ui.Ops
 	constrained bool
-	cs          Constraints
+	ctx         *Context
 	maxSZ       image.Point
 	baseline    int
 }
@@ -30,9 +30,9 @@ type StackChild struct {
 }
 
 // Init a stack before calling Rigid or Expand.
-func (s *Stack) Init(ops *ui.Ops, cs Constraints) *Stack {
+func (s *Stack) Init(ops *ui.Ops, ctx *Context) *Stack {
 	s.ops = ops
-	s.cs = cs
+	s.ctx = ctx
 	s.constrained = true
 	s.maxSZ = image.Point{}
 	s.baseline = 0
@@ -50,7 +50,8 @@ func (s *Stack) begin() {
 // passed to Init.
 func (s *Stack) Rigid(w Widget) StackChild {
 	s.begin()
-	return s.end(w(s.cs))
+	dims := s.ctx.Layout(s.ctx.Constraints, w)
+	return s.end(dims)
 }
 
 // Expand lays out a widget with constraints that exactly match
@@ -61,7 +62,8 @@ func (s *Stack) Expand(w Widget) StackChild {
 		Width:  Constraint{Min: s.maxSZ.X, Max: s.maxSZ.X},
 		Height: Constraint{Min: s.maxSZ.Y, Max: s.maxSZ.Y},
 	}
-	return s.end(w(cs))
+	dims := s.ctx.Layout(cs, w)
+	return s.end(dims)
 }
 
 // End a child by specifying its dimensions.
@@ -83,7 +85,7 @@ func (s *Stack) end(dims Dimensions) StackChild {
 
 // Layout a list of children. The order of the children determines their laid
 // out order.
-func (s *Stack) Layout(children ...StackChild) Dimensions {
+func (s *Stack) Layout(children ...StackChild) {
 	for _, ch := range children {
 		sz := ch.dims.Size
 		var p image.Point
@@ -109,7 +111,7 @@ func (s *Stack) Layout(children ...StackChild) Dimensions {
 	if b == 0 {
 		b = s.maxSZ.Y
 	}
-	return Dimensions{
+	s.ctx.Dimensions = Dimensions{
 		Size:     s.maxSZ,
 		Baseline: b,
 	}
diff --git a/ui/text/editor.go b/ui/text/editor.go
index 5993700c..40a8d504 100644
--- a/ui/text/editor.go
+++ b/ui/text/editor.go
@@ -165,7 +165,8 @@ func (e *Editor) Focus() {
 	e.requestFocus = true
 }
 
-func (e *Editor) Layout(cfg ui.Config, queue input.Queue, ops *ui.Ops, cs layout.Constraints) layout.Dimensions {
+func (e *Editor) Layout(cfg ui.Config, queue input.Queue, ops *ui.Ops, ctx *layout.Context) {
+	cs := ctx.Constraints
 	for _, ok := e.Next(cfg, queue); ok; _, ok = e.Next(cfg, queue) {
 	}
 	twoDp := cfg.Px(ui.Dp(2))
@@ -270,7 +271,7 @@ func (e *Editor) Layout(cfg ui.Config, queue input.Queue, ops *ui.Ops, cs layout
 	pointer.RectAreaOp{Rect: r}.Add(ops)
 	e.scroller.Add(ops)
 	e.clicker.Add(ops)
-	return layout.Dimensions{Size: e.viewSize, Baseline: baseline}
+	ctx.Dimensions = layout.Dimensions{Size: e.viewSize, Baseline: baseline}
 }
 
 // Text returns the contents of the editor.
diff --git a/ui/text/label.go b/ui/text/label.go
index d7d69d19..79a94bd0 100644
--- a/ui/text/label.go
+++ b/ui/text/label.go
@@ -92,7 +92,8 @@ func (l *lineIterator) Next() (String, f32.Point, bool) {
 	return String{}, f32.Point{}, false
 }
 
-func (l Label) Layout(ops *ui.Ops, cs layout.Constraints) layout.Dimensions {
+func (l Label) Layout(ops *ui.Ops, ctx *layout.Context) {
+	cs := ctx.Constraints
 	textLayout := l.Face.Layout(l.Text, LayoutOptions{MaxWidth: cs.Width.Max})
 	lines := textLayout.Lines
 	if max := l.MaxLines; max > 0 && len(lines) > max {
@@ -127,7 +128,7 @@ func (l Label) Layout(ops *ui.Ops, cs layout.Constraints) layout.Dimensions {
 		paint.PaintOp{Rect: lclip}.Add(ops)
 		stack.Pop()
 	}
-	return dims
+	ctx.Dimensions = dims
 }
 
 func toRectF(r image.Rectangle) f32.Rectangle {
diff --git a/ui/widget/image.go b/ui/widget/image.go
index 90dd1599..0a33c6ee 100644
--- a/ui/widget/image.go
+++ b/ui/widget/image.go
@@ -25,7 +25,7 @@ type Image struct {
 	Scale float32
 }
 
-func (im Image) Layout(c ui.Config, ops *ui.Ops, cs layout.Constraints) layout.Dimensions {
+func (im Image) Layout(c ui.Config, ops *ui.Ops, ctx *layout.Context) {
 	size := im.Src.Bounds()
 	wf, hf := float32(size.Dx()), float32(size.Dy())
 	var w, h int
@@ -35,6 +35,7 @@ func (im Image) Layout(c ui.Config, ops *ui.Ops, cs layout.Constraints) layout.D
 	} else {
 		w, h = int(wf*im.Scale+.5), int(hf*im.Scale+.5)
 	}
+	cs := ctx.Constraints
 	d := image.Point{X: cs.Width.Constrain(w), Y: cs.Height.Constrain(h)}
 	aspect := float32(w) / float32(h)
 	dw, dh := float32(d.X), float32(d.Y)
@@ -49,5 +50,5 @@ func (im Image) Layout(c ui.Config, ops *ui.Ops, cs layout.Constraints) layout.D
 	}
 	paint.ImageOp{Src: im.Src, Rect: im.Rect}.Add(ops)
 	paint.PaintOp{Rect: dr}.Add(ops)
-	return layout.Dimensions{Size: d, Baseline: d.Y}
+	ctx.Dimensions = layout.Dimensions{Size: d, Baseline: d.Y}
 }