[Macintosh]QStyle::drawPrimitive(PE_FrameFocusRect, ...) doesn't draw anything on Macintosh

I wanted to draw a focus ring on my custom controls on a Macintosh. So I naively used QStyle::drawPrimitive(PE_FrameFocusRect, ...) to do it based on the documentation for QStyle. It didn't draw anything.

Tracing into the code to figure out why, I find this in QMacStyle::drawPrimitive() (qmacstyle_mac.cpp):

    case PE_FrameFocusRect:
        // Use the our own focus widget stuff.
        break;

In the QStyle documentation, under the heading "Developing Style-Aware Custom Widgets" the example code is drawing a focus rectangle!