[jonathanstrange]

Yes, that's my point. \sloppybottom can look fine but it violates the requirement of real register-true typesetting where each typeblock has the same size on every page and the lines on a double page match exactly. Some publishers have this requirement and it's hard to work around it. This could in principle be improved by some subtle tradeoffs between line breaks in previous paragraphs, paragraph breaks, and page breaks. It's a kind of global optimization that is not possible in TeX due to principal limitations of the engine. See Section 4 of [1].

[1] https://tug.org/TUGboat/tb11-3/tb29mitt.pdf

[pja]

Simon Cozens spent some time writing a new typesetter called SILE:

  https://sile-typesetter.org/

On of the design goals was to be able to achieve exactly that kind of line matching. IIRC it can ensure that lines on the front & back of a page line up exactly too; apparently this is important for bibles?

Worth taking a look at. It recently acquired TeX style mathematics typesetting ability & has a small but active developer group.