message: TreeHeader: hide the separator lines (Qt >= 5.10, macOS)
  See qtbase commit (https://codereview.qt-project.org/#/c/211068/)
diff: @@ -55,8 +55,10 @@ void TreeHeader::paintEvent(QPaintEvent*)
  {
  QStyleOptionHeader option;
  option.init(this);
  -#ifdef Q_OS_WIN
  +#if defined(Q_OS_WIN)
  option.rect.adjust(0, 0, 0, 1);
  +#elif defined(Q_OS_MAC)
  + option.rect.adjust(0, 0, 1, 0);
  #endif
  option.state = (d.state | QStyle::State_Raised | QStyle::State_Horizontal);
  QPixmap px(16, 16);
  