This is a full, fundamental redesign and rewrite of Typst's most central layouter, the flow layouter. This layouter is responsible for arranging paragraphs, blocks, spacing, placed elements, floats, and footnotes across pages.

Why a new flow layouter?

The old flow layouter was written a long time ago and grew over time. New features, like footnotes and floats, were mostly bolted on top: This showed up in a lack of features (e.g. floats over two columns) and bugs (e.g. footnotes in breakable blocks flowing away to the last page of the block). To address those properly required a fundamental shift in how the flow operates. The old flow layout proceeded in a strictly linear manner: It processed elements one by one, with no way to go back and revise a decision. This mode of operation is quite limiting and can fundamentally not handle all features we want Typst to have.

How does it work?

The new flow layout operates in a sort of flipped-around way: Instead of walking elements one by one, it walks pages one by one. For each page, it fills it with content as long as space is available. And here comes the crucial part: While laying out a page, it can decide to trigger a relayout of the page. This happens when a float or footnote is inserted, as this can affect the layout of the page's inner content. I wrote a bit more about this approach in a blog post some time ago: In summary, the approach of gaining information and then relayouting with that new information can help us iteratively solve various problems that can't be solved in a single step. It's sort of similar to how Typst already runs multiple layout iterations to resolve introspection, but additional and add the page-level. (It might seem like a good idea to actually use the normal introspection for these things, but one runs into convergence issues immediately, which is why the iteration scope must be tighter.)

What does it do?

User-facing, this PR

• Adds new page-scoped floating placement
• Adds a new public block.sticky to property to prevent a pagebreak after a block. This is used internally by headings.
• The height of blocks, images, and some shapes can now be specified in fr units.
• Fixes many bugs, the most important ones being:
  • No more orphaned headings at the bottom of the page.
  • Footnotes in breakable blocks now work properly. It can still happen that a footnote is pushed to the next page due to a lack of space, but it's much rarer.

The new floating placement looks like this:

#set page(columns: 2)

// Raw placement.
#place(
  top,
  scope: "page",
  float: true,
  [This spans both columns]
)

// With a figure.
#figure(
  placement: auto,
  scope: "page",
  caption: [My caption],
  rect(width: 75%),
)

#lorem(200)

The PR also lays the groundwork for exciting future features like to-the-side wrapping floats, layout through chained containers, page margins depending on the page number, and more.

Notes

• In my very unscientific testing on a few documents, performance was not affected much. While working on the PR, I primarily on focused on getting things right and not hurting performance too much, so that's a good result I would say. Fundamentally, doing relayouts is of course more expensive than not doing them.

• The PR also adds the new file tests/skip.txt, which can be used to temporarily disable all the tests listed within. This is very useful during large incremental refactors.

Linked issues

• Closes Multi-column Float Elements (Figures, Tables, Code Blocks, etc.) #553
• Closes Keep with the next paragraph #993
• Closes Support fraction as block height #4220
• Fixes Unbreakable block becomes invalid when moved into differently sized region #3866
• Fixes align(horizon) messes with v(1fr) #2213
• Fixes Breakable block skips region due to placed floating element #3907
• Fixes Footnote in block renders at the page where the block ends #2449
• Fixes Headings do not always respect widow and orphan prevention #1044
• Fixes Footnote in auto placed figure causing figure to placed on top of text #4780