Linux: a family name matching a fontconfig substring rule resolves silently to an unrelated face #187

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opened 2026-10-19 12:04:25 +09:00 by natsukium · 0 comments

natsukium commented

2026-10-19 12:04:25 +09:00

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Problem

On Linux, font.family names that are not installed can resolve to an unrelated face with no warning. The unknown-family check in crates/felis-shaping/src/provider/fontconfig.rs (substitutes_as_unknown) calls a name unknown only when fontconfig substitution treats it exactly as it treats a name no rule mentions. Stock fontconfig rules match on part of the family name, so a name that hits one of them counts as known even when no installed family carries it.

Observed on this host (NixOS, stock fontconfig rules), with font.family set to each name:

`font.family`	Warned	Primary drawn
`Qwxyz`	yes	default monospace
`Qwxyz Nerd Font`	yes	default monospace
`Qwxyz Mono`	no	DejaVu Sans Mono
`JetBrainsMono Nerd Fnt` (typo)	no	DejaVu Sans Mono
`Noto Sans Qwxyz`	no	DejaVu Sans (proportional)

fc-match gives the same faces, so felis follows fontconfig here. The result is still wrong for felis's purpose:

The most common typo goes unreported. Nerd Font family names usually contain Mono.
A name starting with Noto Sans makes a proportional face the primary, with no warning.

It is not known yet which rules are responsible; the contains-style tests in the stock config are the first suspects.

Constraint

docs/explanation/rendering/text-shaping.md "Linux faces" records that unknown families are detected by the config rules fontconfig applies, not by comparing names, so that an <alias> mapping a requested name elsewhere is honored. A fix must keep that: an explicit alias for the exact name still counts as known, while a generic rule that matched only part of the name does not.

Possible direction

Treat a name as known when an installed family matches it (fontconfig's comparison: case-insensitive, ignoring blanks), or when substitution put a family other than the generic ones (monospace, sans-serif, serif, and their expansions) ahead of where the unknown sentinel puts them. This needs a design, and the "Linux faces" record updated.

## Problem On Linux, `font.family` names that are not installed can resolve to an unrelated face with no warning. The unknown-family check in `crates/felis-shaping/src/provider/fontconfig.rs` (`substitutes_as_unknown`) calls a name unknown only when fontconfig substitution treats it exactly as it treats a name no rule mentions. Stock fontconfig rules match on part of the family name, so a name that hits one of them counts as known even when no installed family carries it. Observed on this host (NixOS, stock fontconfig rules), with `font.family` set to each name: | `font.family` | Warned | Primary drawn | | --- | --- | --- | | `Qwxyz` | yes | default monospace | | `Qwxyz Nerd Font` | yes | default monospace | | `Qwxyz Mono` | no | DejaVu Sans Mono | | `JetBrainsMono Nerd Fnt` (typo) | no | DejaVu Sans Mono | | `Noto Sans Qwxyz` | no | DejaVu Sans (proportional) | `fc-match` gives the same faces, so felis follows fontconfig here. The result is still wrong for felis's purpose: - The most common typo goes unreported. Nerd Font family names usually contain `Mono`. - A name starting with `Noto Sans` makes a proportional face the primary, with no warning. It is not known yet which rules are responsible; the `contains`-style tests in the stock config are the first suspects. ## Constraint docs/explanation/rendering/text-shaping.md "Linux faces" records that unknown families are detected by the config rules fontconfig applies, not by comparing names, so that an `<alias>` mapping a requested name elsewhere is honored. A fix must keep that: an explicit alias for the exact name still counts as known, while a generic rule that matched only part of the name does not. ## Possible direction Treat a name as known when an installed family matches it (fontconfig's comparison: case-insensitive, ignoring blanks), or when substitution put a family other than the generic ones (`monospace`, `sans-serif`, `serif`, and their expansions) ahead of where the unknown sentinel puts them. This needs a design, and the "Linux faces" record updated.

natsukium added the

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2026-10-19 12:04:29 +09:00

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felis-terminal/felis#187

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