layout: do not reset cross axis constraints in Direction for N, S, E and W

Fixes #268

Signed-off-by: Pierre Curto <pierre.curto@gmail.com>

layout/layout.go

@@ -162,16 +162,23 @@ func UniformInset(v unit.Value) Inset {
 // The widget is called with the context constraints minimum cleared.
 func (d Direction) Layout(gtx Context, w Widget) Dimensions {
 	macro := op.Record(gtx.Ops)
-	cs := gtx.Constraints
-	gtx.Constraints.Min = image.Point{}
+	csn := gtx.Constraints.Min
+	switch d {
+	case N, S:
+		gtx.Constraints.Min.Y = 0
+	case E, W:
+		gtx.Constraints.Min.X = 0
+	default:
+		gtx.Constraints.Min = image.Point{}
+	}
 	dims := w(gtx)
 	call := macro.Stop()
 	sz := dims.Size
-	if sz.X < cs.Min.X {
-		sz.X = cs.Min.X
+	if sz.X < csn.X {
+		sz.X = csn.X
 	}
-	if sz.Y < cs.Min.Y {
-		sz.Y = cs.Min.Y
+	if sz.Y < csn.Y {
+		sz.Y = csn.Y
 	}
 
 	defer op.Save(gtx.Ops).Load()

layout/layout_test.go

@@ -29,3 +29,36 @@ func TestStack(t *testing.T) {
 		t.Errorf("Stack ignored Expanded size, got %v expected %v", got, exp)
 	}
 }
+
+func TestDirection(t *testing.T) {
+	max := image.Pt(100, 100)
+	for _, tc := range []struct {
+		dir Direction
+		exp image.Point
+	}{
+		{N, image.Pt(max.X, 0)},
+		{S, image.Pt(max.X, 0)},
+		{E, image.Pt(0, max.Y)},
+		{W, image.Pt(0, max.Y)},
+		{NW, image.Pt(0, 0)},
+		{NE, image.Pt(0, 0)},
+		{SE, image.Pt(0, 0)},
+		{SW, image.Pt(0, 0)},
+		{Center, image.Pt(0, 0)},
+	} {
+		t.Run(tc.dir.String(), func(t *testing.T) {
+			gtx := Context{
+				Ops:         new(op.Ops),
+				Constraints: Exact(max),
+			}
+			var min image.Point
+			tc.dir.Layout(gtx, func(gtx Context) Dimensions {
+				min = gtx.Constraints.Min
+				return Dimensions{}
+			})
+			if got, exp := min, tc.exp; got != exp {
+				t.Errorf("got %v; expected %v", got, exp)
+			}
+		})
+	}
+}