[behdad]

Alternate feature tables inherit their parameter format from the original feature tag. Skera previously resolved them with a placeholder tag, which made ordinary subsetting fail and instancing silently discard valid parameters. Carry the original tag through GSUB/GPOS subsetting and ownership, and retain names referenced by retained alternate features.

Tests cover size, ss01, and cv01 parameters in both tables, feature-index remapping, full and partial instancing, axis ranges, and partial-then-full instancing. All eight complete HarfBuzz table references now include alternate parameters; ordinary and partial references were refreshed after the upstream HarfBuzz fixes. The original parameter regressions fail on the preceding implementation.

Self-review also found that feature-variation ownership accepted unsupported major versions and could panic while accessing an oversized declared record array on 32-bit hosts. Check the version and array extent before generated range access. The regression fails before the fix and passes on native x86_64 and actual i686. The general generated-reader overflow is addressed separately in #2392.

Validation at this PR: all 530 Skera tests with all features, the i686 malformed-header regression, Clippy with warnings denied, Rust 1.85 all-target/all-feature checks, formatting, and typo checks pass. Downstream propagation checks compilation and the header/feature-parameter regressions at each affected boundary; the final refactored tip also runs the full Skera suite.

Readers use read-fonts directly. write-fonts handles font assembly and owned table rebuilding; Skera retains its direct serializers and repacker. There is no separate generated Skera table tree.

[behdad]

Self-review findings before fixes:

• Passing the original feature tag when resolving alternate tables is the right model. I checked feature-index remapping, size/ssXX/cvXX name closure, and the OFF 2026 section 6.2.11 interaction between legacy alternate parameters and lookup variations. The implementation applies the parameter substitution first and then constructs the lookup set, using the current feature's lookups when ADD_DEFAULT_LOOKUPS is set, as specified.
• The owned FeatureVariations conversion currently accepts an unsupported major version and serializes it as a supported version. The new regression fails before the correction. This should be rejected at the ownership boundary.
• Its legacy array getter is called before validating the encoded count's byte extent. On 32-bit hosts, the generated range calculation can overflow for a tiny malformed table with a very large count. I am adding a bounds check before invoking that getter and checking the case on a 32-bit target.
• The differential test deliberately clears alternate parameters because its older HarfBuzz reference build dropped them. That upstream bug is already fixed by merged HarfBuzz #6349 / #6350, and FontTools #4278. I will regenerate the references and compare the complete parameter-bearing layout tables instead of clearing those fields. There is no need to file duplicate upstream PRs.

[behdad]

Self-review fixes are now validated and propagated through the stack. Feature-variation ownership rejects unsupported major versions and oversized record arrays before generated range access. The malformed-header regression reproduces the original acceptance bug on x86_64 and the original overflow panic on actual i686; both pass after the guard. The general generator/read-fonts fix is in #2392.

The feature-parameter comparisons now check complete alternate parameters against all eight current HarfBuzz references. The previous normalization that stripped those parameters has been removed; the relevant HarfBuzz and FontTools parameter fixes already landed, so no duplicate upstream PR is needed.

This PR passes all 530 Skera tests, Clippy with warnings denied, Rust 1.85 all-target/all-feature checks, formatting, and typo checks. Every affected downstream boundary compiles and passes the ownership/parameter regressions. The final refactored tip passes 627 Skera tests, Clippy, MSRV, formatting, typo checks, and the i686 header regression. All affected branches were pushed atomically after validation. The design document records the ownership boundary and validation scope.

[behdad]

Superseded by the rebuilt, consolidated instancing stack in #2416–#2423. MATH and name-table overrides remain separately covered by #2415 and #2411.