0024 — The page is a fixed box; content is pushed through it

• Status: Accepted
• Date: 2026-08-21
• Supersedes: —
• Amended by: 0025, which withdraws the listing exception below
• Superseded by: —

Context

The page view shows a .tex file set on sheets of paper. Four attempts at it all built the page out of the content: work out how many rows of text a sheet holds, count the rows each line takes, break when the count runs out, and pad the foot of a sheet that came up short.

Each attempt improved the count and none of them fixed the problem, because the count was never the problem.

A page assembled from content can stretch. The padding at the foot only helps a sheet that came up short; there is nothing that can shrink one that ran over. So every under-estimate — an image that turned out to be four hundred pixels tall, a glossary entry whose definition wrapped to three rows, a title set in \LARGE — did not make its sheet slightly wrong. It made the sheet grow, and a sheet that grows is not a sheet.

The reports were all the same shape and all had this cause: a title page twice the height of the paper, a glossary spread over pages of four different lengths, images stretching a sheet instead of moving to the next one, and pages that changed length when the magnification changed.

Measuring the content instead of counting it made the estimate exact and did not help, because an exact height still cannot shrink a page that is made of the thing being measured.

Decision

Two halves, and only one of them decides where a page is.

The page is a fixed box

A sheet is painted behind the text as a repeating gradient: the paper's height, then the gap, repeating. It is computed from the page size the author chose and the magnification they are looking at, and it knows nothing whatever about the content.

It therefore cannot stretch. Every sheet is exactly the same height as every other sheet because they are the same gradient. There is no arrangement of text that can change that, which is the property every earlier version lacked.

Content is pushed through it

A view plugin measures where each block actually sits and, wherever one would cross a sheet's bottom margin, inserts a gap of exactly the height needed to carry it to the top of the next sheet.

Nothing is padded and nothing is estimated. The gap is the difference between two measured numbers, and it only ever moves content down.

Why that settles rather than oscillating

Inserting a gap moves everything below it, which changes what needs a gap. The loop closes because every gap carries a block onto a sheet where it fits, so each pass has strictly less to do than the last, and a block that has been carried to the top of a sheet is never carried again.

A block taller than the usable page is the exception: it cannot be made to fit, so it is left where it is and allowed to overflow — exactly as LaTeX leaves an oversized image sticking off the paper.

Consequences

• A sheet is always the size of the paper. Not usually, not when the estimate is good: always, because it is not made of anything that could change its size.
• The page size is arithmetic, not measurement. A4 is 297 mm whatever the font is doing. Measuring it was a bug of its own: the magnification is a CSS custom property, so reading a line height straight after changing it paired the old height with the new zoom, and every zoom step computed the wrong page.
• What is measured is how tall the content turned out, which is the one thing only the browser knows.
• Only what is on screen is paginated. The rest of a hundred-page document has never been laid out and has no honest height; it keeps the gaps it had and is corrected before it is drawn.
• The row bookkeeping is gone. Recording how tall every line was drawn, how much of it was hidden and how many rows each widget stood for ran on every replacement in the document on every keystroke. Removing it took the decoration pass on a joined thesis from 14.9 ms to 13.0 ms.
• A generated listing still divides itself, because a gap cannot be put inside a widget. Withdrawn by ADR-0025: nothing in the buffer decides how long a contents list is, so the preview draws a card standing in for one and the list itself lives in a tab. Nothing divides itself into sheets now, and the row count is gone.
• The front matter no longer has a short sheet of its own. Every sheet is the same height, so it takes a whole one. That is the price of the property above and it is worth paying.