• [diff] Add new fonttools diff command for comparing font files, imported from the fdiff project and heavily reworked (#1190, #4007, #4009, #4011, #4013, #4019).
• [feaLib] Fix VariableScalar interpolation bug with non-linear avar mappings. Also decouple VariableScalar from compiled fonts, allowing it to work with designspace data before compilation (#3938, #4054).
• [feaLib] Fix VariableScalar axis ordering and iterative delta rounding to match fontc behavior (#4053).
• [feaLib] Merge chained multi subst rules with same context into a single subtable instead of emitting one subtable per glyph (#4016, #4058).
• [feaLib] Pass location to ConditionsetStatement to fix glyphsLib round-tripping (fontra/fontra-glyphs#130, #4057).
• [feaLib] Write 0xFFFF instead of 0 for missing nameIDs in cv feature params (#4010, #4012).
• [cmap] Fix CmapSubtable.__lt__() TypeError on Python 3 when subtables share the same encoding record, and add compile-time validation for unique encoding records (#4035, #4055).
• [svgLib] Skip non-element XML nodes (comments, processing instructions) when drawing SVG paths (#4042, #4043).
• [glifLib] Fix regression reading glyph outlines when glyphObject=None (#4030, #4031).
• [pointPen] Fix SegmentToPointPen edge case: only remove a duplicate final point on closePath() if it is an on-curve point (#4014, #4015).
• [cffLib] SECURITY Replace eval() with safeEval() in parseBlendList() to prevent arbitrary code execution from crafted TTX files (#4039, #4040).
• [ttLib] Remove defunct Adobe SING Glyphlet tables (META, SING, GMAP, GPKG) (#4044).
• [varLib.interpolatable] Various bugfixes: fix swapped nodeTypes assignment, duplicate kink-detector condition, typos, CFF2 vsindex parsing, glyph existence check, and plot helpers (#4046).
• [varLib.models] Fix getSubModel not forwarding extrapolate/axisRanges; check location uniqueness after stripping zeros (#4047).
• [varLib] Fix --variable-fonts filter in build_many; remove dead code and fix comments (#4048).
• [avar] Preserve existing name table in build; keep unbuild return types consistent; validate map CLI coordinates (#4051).
• [cu2qu/qu2cu] Add input validation: reject non-positive tolerances, validate curve inputs and list lengths (#4052).
• [colorLib] Raise a clear ColorLibError when base glyphs are missing from glyphMap, instead of a confusing KeyError (#4041).
• [glyf] Remove unnecessary fvar table dependency (#4017).
• [fvar/trak] Remove unnecessary name table dependency (#4018).
• [ufoLib] Relax guideline validation to follow the updated spec (#3537, #3553).
• [ttFont] Fix saveXML regression with empty table lists, clarify docstring (#4025, #4026, #4056).
• [setup.py] Link libm for Cython extensions using math functions (#4028, #4029).
• Add typing annotations for DSIG, DefaultTable, ttProgram (#4033).