2021-07-01: Aligning \qedhere after TikZ diagram             rak
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Getting the QED tomb properly aligned with the bottom of a
diagram can be annoying. Especially when you have done it in the
past, but have forgotten how. To help future me out, the
solution is to wrap the diagram in a gathered environment with
bottom alignment, and then to add a magic new lign right after
you close the (in this case) tikzcd environment:

    \begin{gathered}[b]
      \begin{tikzcd}
        FX
        \ar[r, "{\eta_X}"]
        \ar[d, swap, "Ff"]
        &
        GX
        \ar[d, "Gf"]
        \\
        FY
        \ar[r, "{\eta_Y}"]
        &
        GY
      \end{tikzcd}\\[-\dp\strutbox]
    \end{gathered}\qedhere