[instancer] Ensure hhea vertical metrics stay in sync with OS/2 ones after instancing

Fixes https://github.com/fonttools/fonttools/issues/3297

Lib/fontTools/varLib/instancer/__init__.py

@@ -105,6 +105,7 @@ from fontTools.misc.cliTools import makeOutputFileName
 from fontTools.varLib.instancer import solver
 import collections
 import dataclasses
+from contextlib import contextmanager
 from copy import deepcopy
 from enum import IntEnum
 import logging
@@ -694,6 +695,43 @@ def setMvarDeltas(varfont, deltas):
             )
 
 
+@contextmanager
+def verticalMetricsKeptInSync(varfont):
+    """Ensure hhea vertical metrics stay in sync with OS/2 ones after instancing.
+
+    When applying MVAR deltas to the OS/2 table, if the ascender, descender and
+    line gap change but they were the same as the respective hhea metrics in the
+    original font, this context manager ensures that hhea metrcs also get updated
+    accordingly.
+    The MVAR spec only has tags for the OS/2 metrics, but it is common in fonts
+    to have the hhea metrics be equal to those for compat reasons.
+
+    https://learn.microsoft.com/en-us/typography/opentype/spec/mvar
+    https://googlefonts.github.io/gf-guide/metrics.html#7-hhea-and-typo-metrics-should-be-equal
+    https://github.com/fonttools/fonttools/issues/3297
+    """
+    current_os2_vmetrics = [
+        getattr(varfont["OS/2"], attr)
+        for attr in ("sTypoAscender", "sTypoDescender", "sTypoLineGap")
+    ]
+    metrics_are_synced = current_os2_vmetrics == [
+        getattr(varfont["hhea"], attr) for attr in ("ascender", "descender", "lineGap")
+    ]
+
+    yield metrics_are_synced
+
+    if metrics_are_synced:
+        new_os2_vmetrics = [
+            getattr(varfont["OS/2"], attr)
+            for attr in ("sTypoAscender", "sTypoDescender", "sTypoLineGap")
+        ]
+        if current_os2_vmetrics != new_os2_vmetrics:
+            for attr, value in zip(
+                ("ascender", "descender", "lineGap"), new_os2_vmetrics
+            ):
+                setattr(varfont["hhea"], attr, value)
+
+
 def instantiateMVAR(varfont, axisLimits):
     log.info("Instantiating MVAR table")
 
@@ -701,7 +739,9 @@ def instantiateMVAR(varfont, axisLimits):
     fvarAxes = varfont["fvar"].axes
     varStore = mvar.VarStore
     defaultDeltas = instantiateItemVariationStore(varStore, fvarAxes, axisLimits)
-    setMvarDeltas(varfont, defaultDeltas)
+
+    with verticalMetricsKeptInSync(varfont):
+        setMvarDeltas(varfont, defaultDeltas)
 
     if varStore.VarRegionList.Region:
         varIndexMapping = varStore.optimize()