diff --git a/ui/app/app.go b/ui/app/app.go
index fcbed16a..887ff58a 100644
--- a/ui/app/app.go
+++ b/ui/app/app.go
@@ -4,8 +4,10 @@ package app
 
 import (
 	"image"
+	"math"
 	"os"
 	"strings"
+	"time"
 
 	"gioui.org/ui"
 )
@@ -15,7 +17,7 @@ type Event interface {
 }
 
 type DrawEvent struct {
-	Config ui.Config
+	Config Config
 	Size   image.Point
 	// Insets is the window space taken up by
 	// system decoration such as translucent
@@ -131,3 +133,34 @@ func init() {
 func DataDir() (string, error) {
 	return dataDir()
 }
+
+// Config implements the ui.Config interface.
+type Config struct {
+	// Device pixels per dp.
+	pxPerDp float32
+	// Device pixels per sp.
+	pxPerSp float32
+	now     time.Time
+}
+
+func (c *Config) Now() time.Time {
+	return c.now
+}
+
+func (c *Config) Px(v ui.Value) int {
+	var r float32
+	switch v.U {
+	case ui.UnitPx:
+		r = v.V
+	case ui.UnitDp:
+		r = c.pxPerDp * v.V
+	case ui.UnitSp:
+		r = c.pxPerSp * v.V
+	default:
+		panic("unknown unit")
+	}
+	if math.IsInf(float64(r), +1) {
+		return ui.Inf
+	}
+	return int(math.Round(float64(r)))
+}
diff --git a/ui/app/os_android.go b/ui/app/os_android.go
index 4fb9a35e..37448dce 100644
--- a/ui/app/os_android.go
+++ b/ui/app/os_android.go
@@ -24,7 +24,6 @@ import (
 	"time"
 	"unsafe"
 
-	"gioui.org/ui"
 	"gioui.org/ui/f32"
 	"gioui.org/ui/key"
 	"gioui.org/ui/pointer"
@@ -289,10 +288,10 @@ func (w *window) draw(sync bool) {
 			Y: int(height),
 		},
 		Insets: w.insets,
-		Config: ui.Config{
-			PxPerDp: ppdp,
-			PxPerSp: w.fontScale * ppdp,
-			Now:     time.Now(),
+		Config: Config{
+			pxPerDp: ppdp,
+			pxPerSp: w.fontScale * ppdp,
+			now:     time.Now(),
 		},
 		sync: sync,
 	})
diff --git a/ui/app/os_ios.go b/ui/app/os_ios.go
index 6fd98b42..d08a0e5b 100644
--- a/ui/app/os_ios.go
+++ b/ui/app/os_ios.go
@@ -22,7 +22,6 @@ import (
 	"sync/atomic"
 	"time"
 
-	"gioui.org/ui"
 	"gioui.org/ui/f32"
 	"gioui.org/ui/key"
 	"gioui.org/ui/pointer"
@@ -87,10 +86,10 @@ func onDraw(view C.CFTypeRef, dpi, sdpi, width, height C.CGFloat, sync C.int, to
 			Min: image.Point{X: int(left + .5), Y: int(top + .5)},
 			Max: image.Point{X: int(right + .5), Y: int(bottom + .5)},
 		},
-		Config: ui.Config{
-			PxPerDp: float32(dpi) * inchPrDp,
-			PxPerSp: float32(sdpi) * inchPrDp,
-			Now:     time.Now(),
+		Config: Config{
+			pxPerDp: float32(dpi) * inchPrDp,
+			pxPerSp: float32(sdpi) * inchPrDp,
+			now:     time.Now(),
 		},
 		sync: isSync,
 	})
diff --git a/ui/app/os_js.go b/ui/app/os_js.go
index 1fcae562..5e3a087e 100644
--- a/ui/app/os_js.go
+++ b/ui/app/os_js.go
@@ -7,7 +7,6 @@ import (
 	"syscall/js"
 	"time"
 
-	"gioui.org/ui"
 	"gioui.org/ui/f32"
 	"gioui.org/ui/key"
 	"gioui.org/ui/pointer"
@@ -329,13 +328,13 @@ func (w *window) setTextInput(s key.TextInputState) {
 
 func (w *window) draw(sync bool) {
 	width, height, scale, cfg := w.config()
-	if cfg == (ui.Config{}) {
+	if cfg == (Config{}) {
 		return
 	}
 	w.mu.Lock()
 	w.scale = float32(scale)
 	w.mu.Unlock()
-	cfg.Now = time.Now()
+	cfg.now = time.Now()
 	w.w.event(DrawEvent{
 		Size: image.Point{
 			X: width,
@@ -346,7 +345,7 @@ func (w *window) draw(sync bool) {
 	})
 }
 
-func (w *window) config() (int, int, float32, ui.Config) {
+func (w *window) config() (int, int, float32, Config) {
 	rect := w.cnv.Call("getBoundingClientRect")
 	width, height := rect.Get("width").Float(), rect.Get("height").Float()
 	scale := w.window.Get("devicePixelRatio").Float()
@@ -359,9 +358,9 @@ func (w *window) config() (int, int, float32, ui.Config) {
 		w.cnv.Set("height", ih)
 	}
 	const ppdp = 96 * inchPrDp * monitorScale
-	return iw, ih, float32(scale), ui.Config{
-		PxPerDp: ppdp * float32(scale),
-		PxPerSp: ppdp * float32(scale),
+	return iw, ih, float32(scale), Config{
+		pxPerDp: ppdp * float32(scale),
+		pxPerSp: ppdp * float32(scale),
 	}
 }
 
diff --git a/ui/app/os_macos.go b/ui/app/os_macos.go
index f7e2b3fb..928d273a 100644
--- a/ui/app/os_macos.go
+++ b/ui/app/os_macos.go
@@ -19,7 +19,6 @@ import (
 	"time"
 	"unsafe"
 
-	"gioui.org/ui"
 	"gioui.org/ui/f32"
 	"gioui.org/ui/key"
 	"gioui.org/ui/pointer"
@@ -142,7 +141,7 @@ func (w *window) draw(sync bool) {
 		return
 	}
 	cfg := getConfig()
-	cfg.Now = time.Now()
+	cfg.now = time.Now()
 	w.setStage(StageRunning)
 	w.w.event(DrawEvent{
 		Size: image.Point{
@@ -154,15 +153,15 @@ func (w *window) draw(sync bool) {
 	})
 }
 
-func getConfig() ui.Config {
+func getConfig() Config {
 	ppdp := float32(C.gio_getPixelsPerDP())
 	ppdp *= monitorScale
 	if ppdp < minDensity {
 		ppdp = minDensity
 	}
-	return ui.Config{
-		PxPerDp: ppdp,
-		PxPerSp: ppdp,
+	return Config{
+		pxPerDp: ppdp,
+		pxPerSp: ppdp,
 	}
 }
 
@@ -216,8 +215,8 @@ func Main() {
 	}
 	cfg := getConfig()
 	opts := singleWindow.opts
-	w := cfg.Pixels(opts.Width)
-	h := cfg.Pixels(opts.Height)
+	w := cfg.Px(opts.Width)
+	h := cfg.Px(opts.Height)
 	title := C.CString(opts.Title)
 	defer C.free(unsafe.Pointer(title))
 	C.gio_main(view, title, C.CGFloat(w), C.CGFloat(h))
diff --git a/ui/app/os_wayland.go b/ui/app/os_wayland.go
index d796ac1b..5e267ec9 100644
--- a/ui/app/os_wayland.go
+++ b/ui/app/os_wayland.go
@@ -19,7 +19,6 @@ import (
 	"unicode/utf8"
 	"unsafe"
 
-	"gioui.org/ui"
 	"gioui.org/ui/f32"
 	"gioui.org/ui/key"
 	"gioui.org/ui/pointer"
@@ -237,8 +236,8 @@ func createNativeWindow(opts *WindowOptions) (*window, error) {
 	C.free(unsafe.Pointer(title))
 
 	_, _, cfg := w.config()
-	w.width = cfg.Pixels(opts.Width)
-	w.height = cfg.Pixels(opts.Height)
+	w.width = cfg.Px(opts.Width)
+	w.height = cfg.Px(opts.Height)
 	if conn.decor != nil {
 		// Request server side decorations.
 		w.decor = C.zxdg_decoration_manager_v1_get_toplevel_decoration(conn.decor, w.topLvl)
@@ -1020,11 +1019,11 @@ func (w *window) updateOutputs() {
 	}
 }
 
-func (w *window) config() (int, int, ui.Config) {
+func (w *window) config() (int, int, Config) {
 	width, height := w.width*w.scale, w.height*w.scale
-	return width, height, ui.Config{
-		PxPerDp: w.ppdp * float32(w.scale),
-		PxPerSp: w.ppsp * float32(w.scale),
+	return width, height, Config{
+		pxPerDp: w.ppdp * float32(w.scale),
+		pxPerSp: w.ppsp * float32(w.scale),
 	}
 }
 
@@ -1036,7 +1035,7 @@ func (w *window) draw(sync bool) {
 		return
 	}
 	width, height, cfg := w.config()
-	if cfg == (ui.Config{}) {
+	if cfg == (Config{}) {
 		return
 	}
 	if animating && w.lastFrameCallback == nil {
@@ -1044,7 +1043,7 @@ func (w *window) draw(sync bool) {
 		// Use the surface as listener data for gio_onFrameDone.
 		C.gio_wl_callback_add_listener(w.lastFrameCallback, unsafe.Pointer(w.surf))
 	}
-	cfg.Now = time.Now()
+	cfg.now = time.Now()
 	w.w.event(DrawEvent{
 		Size: image.Point{
 			X: width,
diff --git a/ui/app/os_windows.go b/ui/app/os_windows.go
index b0985189..152fa7be 100644
--- a/ui/app/os_windows.go
+++ b/ui/app/os_windows.go
@@ -13,7 +13,6 @@ import (
 
 	syscall "golang.org/x/sys/windows"
 
-	"gioui.org/ui"
 	"gioui.org/ui/f32"
 	"gioui.org/ui/key"
 	"gioui.org/ui/pointer"
@@ -220,8 +219,8 @@ func createNativeWindow(opts *WindowOptions) (*window, error) {
 	}
 	defer unregisterClass(cls, hInst)
 	wr := rect{
-		right:  int32(cfg.Pixels(opts.Width)),
-		bottom: int32(cfg.Pixels(opts.Height)),
+		right:  int32(cfg.Px(opts.Width)),
+		bottom: int32(cfg.Px(opts.Height)),
 	}
 	dwStyle := uint32(_WS_OVERLAPPEDWINDOW)
 	dwExStyle := uint32(_WS_EX_APPWINDOW | _WS_EX_WINDOWEDGE)
@@ -419,7 +418,7 @@ func (w *window) draw(sync bool) {
 	w.width = int(r.right - r.left)
 	w.height = int(r.bottom - r.top)
 	cfg := configForDC(w.hdc)
-	cfg.Now = time.Now()
+	cfg.now = time.Now()
 	w.w.event(DrawEvent{
 		Size: image.Point{
 			X: w.width,
@@ -487,16 +486,16 @@ func convertKeyCode(code uintptr) (rune, bool) {
 	return r, true
 }
 
-func configForDC(hdc syscall.Handle) ui.Config {
+func configForDC(hdc syscall.Handle) Config {
 	dpi := getDeviceCaps(hdc, _LOGPIXELSX)
 	ppdp := float32(dpi) * inchPrDp * monitorScale
 	// Force a minimum density to keep text legible and to handle bogus output geometry.
 	if ppdp < minDensity {
 		ppdp = minDensity
 	}
-	return ui.Config{
-		PxPerDp: ppdp,
-		PxPerSp: ppdp,
+	return Config{
+		pxPerDp: ppdp,
+		pxPerSp: ppdp,
 	}
 }
 
diff --git a/ui/gesture/gestures.go b/ui/gesture/gestures.go
index 237de5f9..eb68981e 100644
--- a/ui/gesture/gestures.go
+++ b/ui/gesture/gestures.go
@@ -133,7 +133,7 @@ func (s *Scroll) Dragging() bool {
 	return s.dragging
 }
 
-func (s *Scroll) Scroll(cfg *ui.Config, q input.Events, axis Axis) int {
+func (s *Scroll) Scroll(cfg ui.Config, q input.Events, axis Axis) int {
 	if s.axis != axis {
 		s.axis = axis
 		return 0
@@ -165,15 +165,15 @@ func (s *Scroll) Scroll(cfg *ui.Config, q input.Events, axis Axis) int {
 				break
 			}
 			fling := s.estimator.Estimate()
-			if slop, d := float32(cfg.Pixels(touchSlop)), fling.Distance; d >= slop || -slop >= d {
-				if min, v := float32(cfg.Pixels(minFlingVelocity)), fling.Velocity; v >= min || -min >= v {
-					max := float32(cfg.Pixels(maxFlingVelocity))
+			if slop, d := float32(cfg.Px(touchSlop)), fling.Distance; d >= slop || -slop >= d {
+				if min, v := float32(cfg.Px(minFlingVelocity)), fling.Velocity; v >= min || -min >= v {
+					max := float32(cfg.Px(maxFlingVelocity))
 					if v > max {
 						v = max
 					} else if v < -max {
 						v = -max
 					}
-					s.flinger.Init(cfg.Now, v)
+					s.flinger.Init(cfg.Now(), v)
 				}
 			}
 			fallthrough
@@ -200,7 +200,7 @@ func (s *Scroll) Scroll(cfg *ui.Config, q input.Events, axis Axis) int {
 			v := int(math.Round(float64(val)))
 			dist := s.last - v
 			if e.Priority < pointer.Grabbed {
-				slop := cfg.Pixels(touchSlop)
+				slop := cfg.Px(touchSlop)
 				if dist := dist; dist >= slop || -slop >= dist {
 					s.grab = true
 				}
@@ -210,7 +210,7 @@ func (s *Scroll) Scroll(cfg *ui.Config, q input.Events, axis Axis) int {
 			}
 		}
 	}
-	total += s.flinger.Tick(cfg.Now)
+	total += s.flinger.Tick(cfg.Now())
 	return total
 }
 
diff --git a/ui/layout/layout.go b/ui/layout/layout.go
index aa44ff5d..09c61e97 100644
--- a/ui/layout/layout.go
+++ b/ui/layout/layout.go
@@ -96,14 +96,14 @@ type Insets struct {
 	cs                       Constraints
 }
 
-func (in *Insets) Begin(c *ui.Config, ops *ui.Ops, cs Constraints) Constraints {
+func (in *Insets) Begin(c ui.Config, ops *ui.Ops, cs Constraints) Constraints {
 	if in.begun {
 		panic("must End before Begin")
 	}
-	in.top = c.Pixels(in.Top)
-	in.right = c.Pixels(in.Right)
-	in.bottom = c.Pixels(in.Bottom)
-	in.left = c.Pixels(in.Left)
+	in.top = c.Px(in.Top)
+	in.right = c.Px(in.Right)
+	in.bottom = c.Px(in.Bottom)
+	in.left = c.Px(in.Left)
 	in.begun = true
 	in.ops = ops
 	in.cs = cs
diff --git a/ui/layout/list.go b/ui/layout/list.go
index 0671aa3e..53e4c322 100644
--- a/ui/layout/list.go
+++ b/ui/layout/list.go
@@ -18,7 +18,7 @@ type scrollChild struct {
 }
 
 type List struct {
-	Config             *ui.Config
+	Config             ui.Config
 	Inputs             input.Events
 	Axis               Axis
 	Invert             bool
diff --git a/ui/measure/measure.go b/ui/measure/measure.go
index 8bc66083..923b03b8 100644
--- a/ui/measure/measure.go
+++ b/ui/measure/measure.go
@@ -17,7 +17,7 @@ import (
 )
 
 type Faces struct {
-	Config      *ui.Config
+	Config      ui.Config
 	faceCache   map[faceKey]*textFace
 	layoutCache map[layoutKey]cachedLayout
 	pathCache   map[pathKey]cachedPath
@@ -102,7 +102,7 @@ func (f *Faces) init() {
 }
 
 func (f *textFace) Layout(str string, opts text.LayoutOptions) *text.Layout {
-	ppem := fixed.Int26_6(f.faces.Config.Pixels(f.size) * 64)
+	ppem := fixed.Int26_6(f.faces.Config.Px(f.size) * 64)
 	lk := layoutKey{
 		f:    f.font.Font,
 		ppem: ppem,
@@ -120,7 +120,7 @@ func (f *textFace) Layout(str string, opts text.LayoutOptions) *text.Layout {
 }
 
 func (f *textFace) Path(str text.String) ui.BlockOp {
-	ppem := fixed.Int26_6(f.faces.Config.Pixels(f.size) * 64)
+	ppem := fixed.Int26_6(f.faces.Config.Px(f.size) * 64)
 	pk := pathKey{
 		f:    f.font.Font,
 		ppem: ppem,
diff --git a/ui/text/editor.go b/ui/text/editor.go
index d5a30734..767c1e87 100644
--- a/ui/text/editor.go
+++ b/ui/text/editor.go
@@ -21,7 +21,7 @@ import (
 )
 
 type Editor struct {
-	Config     *ui.Config
+	Config     ui.Config
 	Inputs     input.Events
 	Face       Face
 	Alignment  Alignment
@@ -32,7 +32,7 @@ type Editor struct {
 	Hint         string
 	HintMaterial ui.BlockOp
 
-	oldCfg            ui.Config
+	oldScale          int
 	blinkStart        time.Time
 	focused           bool
 	rr                editBuffer
@@ -73,9 +73,10 @@ func (s ChangeEvent) isEditorEvent() {}
 func (s SubmitEvent) isEditorEvent() {}
 
 func (e *Editor) Next() (EditorEvent, bool) {
-	if cfg := *e.Config; cfg != e.oldCfg {
+	// Crude configuration change detection.
+	if scale := e.Config.Px(ui.Sp(100)); scale != e.oldScale {
 		e.invalidate()
-		e.oldCfg = cfg
+		e.oldScale = scale
 	}
 	sbounds := e.scrollBounds()
 	var smin, smax int
@@ -104,7 +105,7 @@ func (e *Editor) Next() (EditorEvent, bool) {
 		switch {
 		case evt.Type == gesture.TypePress && evt.Source == pointer.Mouse,
 			evt.Type == gesture.TypeClick && evt.Source == pointer.Touch:
-			e.blinkStart = e.Config.Now
+			e.blinkStart = e.Config.Now()
 			e.moveCoord(image.Point{
 				X: int(math.Round(float64(evt.Position.X))),
 				Y: int(math.Round(float64(evt.Position.Y))),
@@ -123,7 +124,7 @@ func (e *Editor) Next() (EditorEvent, bool) {
 		if !ok {
 			break
 		}
-		e.blinkStart = e.Config.Now
+		e.blinkStart = e.Config.Now()
 		switch ke := ke.(type) {
 		case key.FocusEvent:
 			e.focused = ke.Focus
@@ -153,7 +154,7 @@ func (e *Editor) Next() (EditorEvent, bool) {
 }
 
 func (e *Editor) caretWidth() fixed.Int26_6 {
-	oneDp := e.Config.Dp(1)
+	oneDp := e.Config.Px(ui.Dp(1))
 	return fixed.Int26_6(oneDp * 64)
 }
 
@@ -167,7 +168,7 @@ func (e *Editor) Layout(ops *ui.Ops, cs layout.Constraints) layout.Dimens {
 			break
 		}
 	}
-	twoDp := e.Config.Dp(2)
+	twoDp := e.Config.Px(ui.Dp(2))
 	e.padLeft, e.padRight = twoDp, twoDp
 	maxWidth := cs.Width.Max
 	if e.SingleLine {
@@ -225,7 +226,7 @@ func (e *Editor) Layout(ops *ui.Ops, cs layout.Constraints) layout.Dimens {
 		ui.PopOp{}.Add(ops)
 	}
 	if e.focused {
-		now := e.Config.Now
+		now := e.Config.Now()
 		dt := now.Sub(e.blinkStart)
 		blinking := dt < maxBlinkDuration
 		const timePerBlink = time.Second / blinksPerSecond
diff --git a/ui/ui.go b/ui/ui.go
index b17929c0..2a7af305 100644
--- a/ui/ui.go
+++ b/ui/ui.go
@@ -11,41 +11,13 @@ import (
 	"gioui.org/ui/internal/ops"
 )
 
-// Config contains the essential configuration for
+// Config represents the essential configuration for
 // updating and drawing a user interface.
-type Config struct {
-	// Device pixels per dp.
-	PxPerDp float32
-	// Device pixels per sp.
-	PxPerSp float32
-	// The current time for animation.
-	Now time.Time
-}
-
-// Dp converts a value in dp units to pixels.
-func (c *Config) Dp(dp float32) int {
-	return c.Pixels(Dp(dp))
-}
-
-// Sp converts a value in sp units to pixels.
-func (c *Config) Sp(sp float32) int {
-	return c.Pixels(Sp(sp))
-}
-
-// Pixels converts a value to pixels.
-func (c *Config) Pixels(v Value) int {
-	var r float32
-	switch v.U {
-	case UnitPx:
-		r = v.V
-	case UnitDp:
-		r = c.PxPerDp * v.V
-	case UnitSp:
-		r = c.PxPerSp * v.V
-	default:
-		panic("unknown unit")
-	}
-	return int(math.Round(float64(r)))
+type Config interface {
+	// Now returns the current animation time.
+	Now() time.Time
+	// Px converts a Value to pixels.
+	Px(v Value) int
 }
 
 // LayerOp represents a semantic layer of UI.
diff --git a/ui/widget/image.go b/ui/widget/image.go
index dc706b56..9f3a4252 100644
--- a/ui/widget/image.go
+++ b/ui/widget/image.go
@@ -23,13 +23,13 @@ type Image struct {
 	Scale float32
 }
 
-func (im Image) Layout(c *ui.Config, ops *ui.Ops, cs layout.Constraints) layout.Dimens {
+func (im Image) Layout(c ui.Config, ops *ui.Ops, cs layout.Constraints) layout.Dimens {
 	size := im.Src.Bounds()
 	wf, hf := float32(size.Dx()), float32(size.Dy())
 	var w, h int
 	if im.Scale == 0 {
 		const dpPrPx = 160 / 72
-		w, h = c.Dp(wf*dpPrPx), c.Dp(hf*dpPrPx)
+		w, h = c.Px(ui.Dp(wf*dpPrPx)), c.Px(ui.Dp(hf*dpPrPx))
 	} else {
 		w, h = int(wf*im.Scale+.5), int(hf*im.Scale+.5)
 	}