Sanitize terminal widget text

uiterm/ui_regression_test.go

@@ -32,3 +32,14 @@ func TestCloseIsNonblockingAndIdempotent(t *testing.T) {
 		t.Fatal("Close did not signal shutdown")
 	}
 }
+
+func TestSafeRuneRemovesTerminalControlCharacters(t *testing.T) {
+	for _, r := range []rune{'\x1b', '\x7f', '\u202e'} {
+		if got := safeRune(r); got != ' ' {
+			t.Errorf("safeRune(%U) = %U, want space", r, got)
+		}
+	}
+	if got := safeRune('A'); got != 'A' {
+		t.Fatalf("safeRune altered printable text: %U", got)
+	}
+}