Fix center-alignment of border/preview label

Fix #3421

src/terminal.go

@@ -781,7 +781,7 @@ func (t *Terminal) ansiLabelPrinter(str string, color *tui.ColorPair, fill bool)
 				window.CPrint(*color, str)
 			}
 		}
-		return printFn, len(text)
+		return printFn, length
 	}
 
 	// Printer that correctly handles ANSI color codes and tab characters

src/util/util_test.go

@@ -184,3 +184,10 @@ func TestRepeatToFill(t *testing.T) {
 		t.Error("Expected:", strings.Repeat("abcde", 4)+"abcde"[:2])
 	}
 }
+
+func TestStringWidth(t *testing.T) {
+	w := StringWidth("─")
+	if w != 1 {
+		t.Errorf("Expected: %d, Actual: %d", 1, w)
+	}
+}