Make terminal UI shutdown idempotent

uiterm/ui_regression_test.go

@@ -0,0 +1,16 @@
+package uiterm
+
+import "testing"
+
+// Regression: Close sent to a bounded channel and could block or enqueue
+// duplicate shutdowns when called more than once.
+func TestCloseIsNonblockingAndIdempotent(t *testing.T) {
+	ui := New(nil)
+	ui.Close()
+	ui.Close()
+	select {
+	case <-ui.close:
+	default:
+		t.Fatal("Close did not signal shutdown")
+	}
+}