Z-fighting is a completely different issue from aliasing. Maybe you need to set/check/increase the QSurfaceFormat::depthSize property (I've no idea what the default is).
If your z-buffer precision is as good as it can be, glPolygonOffset and/or controlling draw-order are next lines of defence against z-fighting.
I seem to remember the old forum allowed image upload; but maybe I'm thinking of the Jira (bugtracker). The markdown formatting here would let you link to a URL hosted elsewhere though (and maybe include it inline).
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