feat(tracker): implement edit modes, resembling tab stops

tracker/surface.go

@@ -0,0 +1,51 @@
+package tracker
+
+import (
+	"image/color"
+
+	"gioui.org/layout"
+	"gioui.org/op/clip"
+	"gioui.org/op/paint"
+)
+
+type Surface struct {
+	Gray    int
+	Inset   layout.Inset
+	FitSize bool
+	Focus   bool
+}
+
+func (s Surface) Layout(gtx C, widget layout.Widget) D {
+	bg := func(gtx C) D {
+		grayInt := s.Gray
+		if s.Focus {
+			grayInt += 8
+		}
+		var grayUint8 uint8
+		if grayInt < 0 {
+			grayUint8 = 0
+		} else if grayInt > 255 {
+			grayUint8 = 255
+		} else {
+			grayUint8 = uint8(grayInt)
+		}
+		color := color.NRGBA{R: grayUint8, G: grayUint8, B: grayUint8, A: 255}
+		paint.FillShape(gtx.Ops, color, clip.Rect{
+			Max: gtx.Constraints.Min,
+		}.Op())
+		return D{Size: gtx.Constraints.Min}
+	}
+	fg := func(gtx C) D {
+		return s.Inset.Layout(gtx, widget)
+	}
+	if s.FitSize {
+		return layout.Stack{}.Layout(gtx,
+			layout.Expanded(bg),
+			layout.Stacked(fg),
+		)
+	}
+	gtxbg := gtx
+	gtxbg.Constraints.Min = gtxbg.Constraints.Max
+	bg(gtxbg)
+	return fg(gtx)
+}