QHeaderView on macOS Mojave is too narrow and does not paint the top and bottom horizontal lines found on native tree views.

A minimum working example in PySide2 is:

import sys
from PySide2 import QtWidgets

app = QtWidgets.QApplication(sys.argv)

tree_widget = QtWidgets.QTreeWidget()
tree_widget.setColumnCount(3)

layout = QtWidgets.QGridLayout()
layout.addWidget(tree_widget)

window = QtWidgets.QWidget()
window.setLayout(layout)
window.show()

sys.exit(app.exec_())

This can be fixed with the following style sheet, which gives approximately the right appearance: 

QHeaderView {
   border: 1px solid rgb(183, 183, 183);
   background: rgb(240, 240, 240);
   border-left: 0px; border-right: 0px;
   font-size: 11pt;
}

QHeaderView::section {
   height: 18;
   border: 0px;
   background: rgb(240, 240, 240);
   padding-left: 10px;
}

QHeaderView::section:horizontal{
   margin-top: 3px;
   margin-bottom: 3px;
   border-right: 1px solid rgb(212, 212, 212);
}