Type: Top-to-Top Leading — leading measured from em box top to em box top

[imachus]

What and why

Japanese layout measures leading from the top of one line's ideographic
em box to the next (the right side, in vertical type), not from baseline
to baseline. The Paragraph panel menu now offers both:

• ParaStyle.leading_model: RomanBaseline (as before, the default) or
  EmBoxTop. With EmBoxTop a line's leading is the space below it — the
  next line's em box top is this line's plus its leading, and the next
  baseline hangs from there by that line's tallest em box — and area
  type's first line touches the top of the frame (its em box top on the
  frame's top, at least Area Type Options' minimum). With baseline
  leading a line's leading stays the space above it.
• Lines of one size are exactly as far apart either way; the models
  differ for mixed sizes and for the first line of area type (fonts
  whose ascent isn't the em box top: Hiragino's nearly is).
• The em box top comes from the font's vertical metrics (Type: vertical type takes upright glyphs' cells from the font's vertical metrics #343), else
  0.88 em. Written only when EmBoxTop.
• text.setFormat {leadingModel: "romanBaseline"|"emBoxTop"}; menu items
  Top-to-Top Leading / Bottom-to-Bottom Leading (checked as set); catalog
  rows in ja and zh-hant.

The default stays baseline to baseline, so no existing or new document changes unless the paragraph is set to Top-to-Top. Japanese layout apps usually default to Top-to-Top for Japanese type; whether VectorCraft's new type should too (it would move area type's first line for every font whose ascent isn't its em box top) is left for a separate decision.

How it was tested

• Full workspace cargo test passes (3,573), with new tests: area type's first line flush with the frame top; lines of one size equally spaced in both models; 40 pt over 20 pt spaced by the big line's leading (Top-to-Top) vs the small line's (baseline); text.setFormat, its error, undo, and saved only when Top-to-Top.
• Rendered horizontal and vertical area type in Hiragino Mincho both ways.
• cargo fmt --check, cargo clippy --workspace --all-targets -D warnings, cargo xtask brands, layers, assets pass; the web build compiles.

Checklist

• cargo xtask ci passes (fmt, clippy, tests, assets, brands, layers, wasm).
• No panics in shipped code.
• Clean-room: behaviour from public docs and JLREQ; feature names only.
• No new assets.

🤖 Generated with Claude Code

[bflatastic]

Thank you @imachus, merged! Top-to-Top Leading is clean, and the file format stays readable by older builds. I rebased it on main, fitting the field into the new ParaStyleFile (the Justify Auto back-compat wrapper), and added one commit on top:

• Speed: each font face now reads its ideographic em box once instead of once per glyph on every layout. It was on the hot path even with the default model.
• Menu: the two leading items show in the Paragraph panel menu only with Show East Asian Options, as in Illustrator, and Reset Panel goes back to Bottom-to-Bottom.
• Saved key: the attribute is saved as leading_model like the other paragraph fields. The command param stays leadingModel.
• Translations: Simplified Chinese and Spanish rows, since those catalogs landed in the meantime.

Checked live: Top-to-Top moves area type's first line so its em box touches the frame top, at the same line pitch. Over MCP, text.setFormat {leadingModel: "emBoxTop"} works with undo/redo, and bad values return clean errors.