[fonttools/fonttools] 577098: [varLib.instancer] Preserve layout ppem device cor...

[Behdad Esfahbod]

Branch: refs/heads/instancer-layout-hint-devices
Home: https://github.com/fonttools/fonttools
Commit: 5770981f1517503141e570cb5264946a9d6416ef
https://github.com/fonttools/fonttools/commit/5770981f1517503141e570cb5264946a9d6416ef
Author: Behdad Esfahbod <beh...@behdad.org>
Date: 2026-10-10 (Sat, 10 Oct 2026)

Changed paths:
M Lib/fontTools/varLib/merger.py
M Tests/varLib/instancer/instancer_test.py
M Tests/varLib/instancer/jstf_test.py

Log Message:
-----------
[varLib.instancer] Preserve layout ppem device corrections

MutatorMerger asserted that every GDEF, BASE, and GPOS device was a
VariationIndex. Ordinary ppem devices in valid variable fonts therefore
failed instancing, including mixed ppem and variation anchors.

Fold only VariationIndex devices and preserve ordinary hint corrections.
Keep Anchor format 3 when either axis still has a ppem device, while
removing resolved variation references on the other axis.

All 36 regression cases fail before the fix. Validated with 612 related
tests; 11 optional tests skipped.

Assisted-by: Codex



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