Right now, the content panel of an IndexLayout is being displayed under its menu panel when it is nested in a PageLayout.
The line being overridden is in oojs-ui/src/styles/layouts/IndexLayout.less :
> .oo-ui-menuLayout-content { top: 3em; }
This line is being overridden by
.oo-ui-menuLayout.oo-ui-menuLayout-showMenu.oo-ui-menuLayout-before .oo-ui-menuLayout-content { top: 0 !important; right: 0 !important; bottom: 0 !important; }