[instancer] drop explicit no-op axes from tuplevariations

Fixes #3453

Lib/fontTools/varLib/instancer/__init__.py

@@ -531,9 +531,13 @@ def changeTupleVariationsAxisLimits(variations, axisLimits):
 def changeTupleVariationAxisLimit(var, axisTag, axisLimit):
     assert isinstance(axisLimit, NormalizedAxisTripleAndDistances)
 
-    # Skip when current axis is missing (i.e. doesn't participate),
+    # Skip when current axis is missing or peaks at 0 (i.e. doesn't participate)
     lower, peak, upper = var.axes.get(axisTag, (-1, 0, 1))
     if peak == 0:
+        # explicitly defined, no-op axes can be omitted
+        # https://github.com/fonttools/fonttools/issues/3453
+        if axisTag in var.axes:
+            del var.axes[axisTag]
         return [var]
     # Drop if the var 'tent' isn't well-formed
     if not (lower <= peak <= upper) or (lower < 0 and upper > 0):

Tests/varLib/instancer/instancer_test.py

@@ -2015,6 +2015,19 @@ class LimitTupleVariationAxisRangesTest:
                 0.5,
                 [TupleVariation({"wght": (1.0, 1.0, 1.0)}, [100, 100])],
             ),
+            # test case from https://github.com/fonttools/fonttools/issues/3453
+            (
+                TupleVariation(
+                    {
+                        "wght": (0.0, 1.0, 1.0),
+                        "ital": (0.0, 0.0, 1.0),  # no-op axis gets dropped
+                    },
+                    [100, 100],
+                ),
+                "ital",
+                0.0,
+                [TupleVariation({"wght": (0.0, 1.0, 1.0)}, [100, 100])],
+            ),
         ],
     )
     def test_positive_var(self, var, axisTag, newMax, expected):