highlight parent page menu item when in custom post type

You should be able to use a body class to style your menu item :

.single-project .menu-item-45 a

In “Appearance > Menu”, you could also add custom CSS classes to menu items. You could give your Portfolio menu item a class of “portfolio” and do this :

.single-project .menu .portfolio a

I do this mainly because I don’t like .menu-item-[number], it’s more readable with a plain text name, but the first solution should work anyway.

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