feat(html): let a host put a floor under the margin a view keeps

[andiwand]

🤖 Generated with Claude Code

HtmlConfig::min_content_margin states, per side, the least distance the generated content keeps from the view's border.

Why a floor and not a value

Every view already insets its content by an amount it names itself — 3mm for a reflowed text document, 16px for the page column and the source views, 8px for a font specimen, nothing for an image or a file listing. No single default could stand in for all of them, so the config is a floor: an unset side leaves the view exactly where it is today, a set side only ever raises it.

HtmlConfig config;
config.min_content_margin.left = Measure("12px");   // never closer than 12px
config.min_content_margin.top = Measure("1cm");

How

The arithmetic is css's. Each inset becomes max(own, var(--odr-min-margin-*, 0px)), so the view's own measure stays in the stylesheet and nothing has to be converted to state it:

.odr-text-flow{padding:max(3mm,var(--odr-min-margin-top,0px)) …}
.odr-xml{padding:max(16px,var(--odr-min-margin-top,0px)) …}

write_content_margin_style declares the variables per view and writes nothing at all when no side is set — that is what keeps the shipped stylesheets independent of the config, which they must be: with embed_shipped_resources=false they are shared under one name across documents.

One place still needs the number in pixels: the gutter around the page column is part of the width the view is fitted to, so page_column_gutter_pixels now reads the config. A side css can read but css_pixels cannot convert (em, %) keeps the built-in gutter — the margin still applies, only the fit cannot count it. A unit css could not read at all is dropped rather than written into the <style> element it would otherwise close.

view	today	with the floor
reflowed text document	3mm	max(3mm, …)
page column (text, slides, drawings, pdf)	16px	max(16px, …), and the fit follows
plain-text view	16px	left on the gutter column, right on the body
xml source view	16px	max(16px, …)
file listing, image view	none	the floor, 0 by default
font specimen	8px	max(8px, …)

A sheet and the media view are not inset: the one meets the edge by design (its ruler is sticky against the scrollport), the other is full-bleed.

Bindings

DirectionalStyle<Measure> was already mirrored everywhere, so the surface is small: python min_content_margin, jni minContentMargin (DirectionalMeasure), wasm minContentMargin (css lengths as strings), apple minContentMargin — where ODRMeasure and ODRDirectionalMeasure gain the initialisers a caller needs to state one, having been read-only style mirrors until now.

Verification

• odr_test: 951 non-output tests and 278 output tests pass; three new html.* tests cover written/not-written, the dropped unit, and the fit.
• jni minContentMarginReachesTheHtml, a pytest, a wasm node --test case and an XCTest cover each binding.
• Default config moves no pixel: every reference document renders byte-identical; the six stylesheets that did change (three shipped, three written inline by the image, pdf and specimen views) were compared with compare-html --driver chrome — 410/411 public and 1383/1403 private matched, and every mismatch reproduces when the reference output is compared against a byte-changed copy of itself, i.e. they are screenshot flakes in the pdf and svg views, not this change.

Not in this PR

A margin reference-output variant. compare-html fails on an output directory the reference repo does not have, so pinning the floor visually needs the reference output regenerated and test/data.cmake advanced — say the word and it goes on this branch.