What is the behaviour of \fontsize if baselineskip is below a certain value?

Question

during my optimization process I found something spurious I cannot explain to myself and which I would like to understand. So with \fontsize you set the fontsize and the baselineskip. However the behaviour of the second paramater is a bit unclear to me, because if you reduce it too much, at a certain point the baselineskip gets bigger again. Let me show you my example:

\documentclass[landscape]{article}
\usepackage[margin=1cm]{geometry}
\usepackage{lipsum,multicol}

\begin{document}
\begin{multicols}{4}
    \fontsize{10}{11}\selectfont
    \lipsum[1]
    \columnbreak
    
    \fontsize{10}{10}\selectfont
    \lipsum[1]
    \columnbreak
    
    \fontsize{10}{9}\selectfont
    \lipsum[1]
    \columnbreak
    
    \fontsize{10}{8}\selectfont
    \lipsum[1]
    \columnbreak
\end{multicols}
\end{document}

If you compile this, your result will be looking like this:

Now from the first to the third column it seems logic, that the baselineskip is reduced and hence less space is needed vertically to typeset. But with the third column, even though the baselineskip is reduced even further, it needs more vertical space overall. It is strange however, that for example from line 11 to 12 (starting with tus and lum), the spacing seems to be narrower than in column 3, while in a lot of line the spacing is bigger. Changing the baselineskip to lower values down to 0 does not modify the vertical alignment further.

What is the purpose of this behaviour and is it intended? I cannot see a benefit of this and I am a bit confused.

Answer 1

If you add

 \lineskiplimit = -\maxdimen

you get the above.

TeX tries to maintain an even baseline spacing, as set by \baselineskip (which is set by the second argument of \fontsize and other places). However normally if the text in the lines is too large (say an inline math fraction) you do not want to maintain the even spacing and over-print the surrounding lines, so if the space that needs to be added gets below \lineskiplimit TeX gives up even spacing and just inserts the fixed space \lineskip between the lines. By default \lineskip is 1pt and \lineskiplimit is 0pt.

By setting a suitable large negative value for \lineskiplimit you can force even spacing even at the expense of over-printing.

In the last column you can see: