sidebar stuck on posts

When I do a clean install with twentyseventeen as theme, and click on a post, the sidebar goes away and the post fills the whole width. This is what I expect and want.

I am working on a twentyseventeen-child theme, and noticed that I had broken this function (the sidebar was stuck on posts as well as the homepage).

My problem is, even when I revert to twentyseventeen original theme, the problem remains. Even when I remove the twentyseventeen-child folder.

I encountered it on my pc. Then replicated it remotely with a new install.

What is going on, how do I fix it (other than a completely new install)?

Thanks.

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