How do I assign a default menu for all articles ?
For example, I could create a hidden menu (like "Single Article" but actually not for any single article) and use its
Itemid at the end of each article's link.
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If your goal is to load a module for only certain articles, there are two easy ways to do this:
Put them all in the same category and then create a hidden menu item that loads the list of all articles in the category. No one ever needs to see the page, it just needs to exist. Then if you publish a module to that menu item, it SHOULD appear on all articles in that category (there may be times when that isn't true).
Put all of them in the same category, install Advanced Module manager, create your module and assign it to that category.