Line length exceeding --columns

[bpj]

When editing in terminal Vim under Termux on my phone the width of the screen/virtual line length is limited to 30 columns, and I don't want physical lines to exceed 60 columns because longer physical lines tend to not scroll smoothly. Since I quite often reformat text by running it through pandoc with -f markdown -t markdown¹ the behavior of Pandoc's --columns option becomes crucial.

Quite understandably with --columns=60 lines are sometimes shorter than 60 columns due to the presence of long words, which I generally find OK but often they are a bit longer which I find annoying. Generally setting --columns=55 leads to better results but I sometimes get lines longer than 60 columns anyway.

So my question is how is line breaking coded to behave? How (much) is it allowed to make lines shorter or longer than the value of --columns? Is it by a fixed amount or by a percentage? Are certain character classes ignored? I'm afraid that I don't see the pattern!

NB I'm not (yet) calling for a way to prevent overlong lines, just probing for my ideal --columns value. So far I have found that results with values between 50 and 55 don't seem to differ substantially from results with 55 while results with 50 results in too many lines which are too short for my liking.

Footnotes

1. Actually using a Vim user command which invokes pandoc using a certain defaults file. ↩

[jgm]

This is all done by the jgm/doclayout library.
The relevant code is at https://github.com/jgm/doclayout/blob/master/src/Text/DocLayout.hs#L550-L561
As far as I can see, this should not produce lines longer than --columns, except in cases where it's impossible to make a break. If you have a short reproducible test case we could examine it further.