Hello @Constantin, Adrian,
Welcome to Microsoft Q&A forum.
Document Outline window is usually used in conjunction with designer views, such as the designer for a XAML page or a Windows Form designer, or with HTML pages. Are you using the corresponding project templates?
Is there a way to see the tags list (functions and global declarations) for the current file in a side-panel?
Does View => Class View meet your requirements? There are also some other windows/Views in VS, you may check if one of them meets your requirements by clicking View/View - Other Windows.
BTW, I think Navigation bar and Solution Explorer have parts of the feature that you want.
Sincerely,
Tianyu
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