QMacStyle::drawPrimitive(QStyle::PE_IndicatorBranch, ...) doesn't honor Qt::RightToLeft option. Using Qt 4.8.6 if you use Qt::RightToLeft on a closed disclosure triangle, it draws the arrow pointing right to left. This appears to have been forgotten in Qt 5.
See qmacstyle_mac.mm, QMacStyle::drawPrimitive(). In Qt 4.8.6:
The same case in Qt 5.4.0 lacks anything like this.
I have tested Qt 5.4.0, and I don't know about earlier versions.