ui: Fix background color of exit node status. (#75)

Prior to https://github.com/tailscale/tailscale-android/pull/73/,
the exit node status pane was set to a background color using:
    paint.Fill(gtx.Ops, bg)

paint.Fill() is documented to fill the entire clipped area.
It assumes that one has already applied a clip area... but
no clip area had been set in this code path.

As far as I can tell, that this worked prior to pull #73
was a bug, something had a side-effect of setting a clipping
rectangle.

We updated to the head of the Gio repo, which apparently fixed
that bug. After pull #73, the paint.Fill() painted the entire
app window.

Fix this using a stacked layout. A color filled widget is the
lower layer, and will expand to the size of the widget sitting
atop of it.

Fixes https://github.com/tailscale/tailscale/issues/6873

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

cmd/tailscale/ui.go

@@ -668,9 +668,20 @@ func (ui *UI) layoutExitStatus(gtx layout.Context, state *BackendState) layout.D
 		text = "Using exit node"
 		bg = rgb(0x338b51)
 	}
-	paint.Fill(gtx.Ops, bg)
 	return material.Clickable(gtx, &ui.openExitDialog, func(gtx C) D {
 		gtx.Constraints.Min.X = gtx.Constraints.Max.X
+		return layout.Stack{}.Layout(gtx,
+			layout.Expanded(func(gtx layout.Context) layout.Dimensions {
+				sz := image.Point{
+					X: gtx.Constraints.Min.X,
+					Y: gtx.Constraints.Min.Y,
+				}
+				defer clip.Rect(image.Rectangle{Max: sz}).Push(gtx.Ops).Pop()
+				paint.ColorOp{Color: bg}.Add(gtx.Ops)
+				paint.PaintOp{}.Add(gtx.Ops)
+				return layout.Dimensions{Size: sz}
+			}),
+			layout.Stacked(func(gtx layout.Context) layout.Dimensions {
 				return layout.Inset{
 					Top:    unit.Dp(12),
 					Bottom: unit.Dp(12),
@@ -697,6 +708,8 @@ func (ui *UI) layoutExitStatus(gtx layout.Context, state *BackendState) layout.D
 						}),
 					)
 				})
+			}),
+		)
 	})
 }