Not properly centering

example.go

package main

import (
	"image"
	"image/color"

	"gioui.org/app"
	"gioui.org/f32"
	"gioui.org/font/gofont"
	"gioui.org/io/system"
	"gioui.org/layout"
	"gioui.org/op/clip"
	"gioui.org/op/paint"
	"gioui.org/unit"
	"gioui.org/widget/material"
)

func main() {
	gofont.Register()

	go func() {
		w := app.NewWindow(
			app.Size(unit.Dp(400), unit.Dp(400)),
			app.Title("C"),
		)
		theme := material.NewTheme()

		gtx := layout.NewContext(w.Queue())

		for {
			e := <-w.Events()
			switch e := e.(type) {
			case system.DestroyEvent:
				panic(e.Err)
			case system.FrameEvent:
				gtx.Reset(e.Config, e.Size)
				// Do layout

				size := 100
				gtx.Constraints.Height.Min = size
				gtx.Constraints.Width.Min = size

				layout.Stack{Alignment: layout.Center}.Layout(gtx,
					layout.Expanded(func() {
						size := float32(gtx.Constraints.Width.Min)
						rr := float32(size) * .5
						clip.Rect{
							Rect: f32.Rectangle{Max: f32.Point{X: size, Y: size}},
							NE:   rr, NW: rr, SE: rr, SW: rr,
						}.Op(gtx.Ops).Add(gtx.Ops)
						fill(gtx, color.RGBA{0, 255, 0, 255})
					}),
					layout.Stacked(func() {
						lbl := theme.Label(unit.Dp(32), "a")
						lbl.Layout(gtx)
					}),
				)

				// End layout

				e.Frame(gtx.Ops)
			}
		}

	}()
	app.Main()
}

func fill(gtx *layout.Context, col color.RGBA) {
	cs := gtx.Constraints
	d := image.Point{X: cs.Width.Min, Y: cs.Height.Min}
	dr := f32.Rectangle{
		Max: f32.Point{X: float32(d.X), Y: float32(d.Y)},
	}
	paint.ColorOp{Color: col}.Add(gtx.Ops)
	paint.PaintOp{Rect: dr}.Add(gtx.Ops)
	gtx.Dimensions = layout.Dimensions{Size: d}
}