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fda75e9af7
dompdf does not use a declared line-height directly. It scales it by the font's
own height:
rendered = declared x (ascent + descent) / unitsPerEm x font_height_ratio
The bundled Noto Sans reports 1.362 for that middle term, so at dompdf's stock
font_height_ratio of 1.1 every line-height in every document came out 1.4985x
what the CSS asked for. Chromium honours the declared value exactly. That one
factor was the whole vertical disagreement between the two drivers.
Setting the ratio to 1/1.362 cancels the font term. Measured on a declared 15px
(11.25pt): 16.86pt at the stock 1.1, 15.32pt at 1.0, and 11.25pt at this value --
identical to Chromium.
Across the seven document templates the worst-edge ink difference falls from
roughly 70-150pt to under 30pt on six of them, and to 3.4pt on invoice1. The
exception is estimate1, which moves the other way: with the line-height noise
gone, a float and padding difference in its address block is now the dominant
term there. That is a separate problem this exposes rather than causes.
Worth recording that an earlier compensation shim had arrived at 1.5 empirically
and was right: 1.1 x 1.362 = 1.4985. I argued against it on the strength of a
test that used font-family: sans-serif, which resolves to a built-in core font
and so never exercised the embedded Noto Sans path where the scaling happens.
The measurement was wrong, not the constant.
PdfLineHeightTest pins the invariant -- the font's reported height equals the
font size, so a declared length renders at that length -- and needs no Gotenberg,
so CI holds it. Swapping the default face or taking a dompdf upgrade that changes
the computation now fails a test rather than quietly reintroducing the drift.
Claude-Session: https://claude.ai/code/session_01QmECndmNZwzN65Zz9P87dF