[c-smile]

12 years ago at W3C styles I've proposed to solve it as simple as this:

   .in-the-middle {
     width:max-content;  
     height:auto;
     margin:*; // "coils" on all sides moving it 
               // to the middle of free space in container
   }

So these tons of tools, essays and doctoral dissertations on "how to center stuff" would not be needed.

Here is the story: https://terrainformatica.com/2018/12/11/10-years-of-flexboxi...