[instancer.solver] Fix case where axisDef < lower and upper < axisMax

Fixes https://github.com/fonttools/fonttools/issues/3291

Lib/fontTools/varLib/instancer/solver.py

@@ -99,11 +99,13 @@ def _solve(tent, axisLimit, negative=False):
     #                axisDef    |  axisMax
     #                           |
     #                      crossing
-    if gain > outGain:
+    if gain >= outGain:
+        # Note that this is the branch taken if both gain and outGain are 0.
+
         # Crossing point on the axis.
         crossing = peak + (1 - gain) * (upper - peak)
 
-        loc = (axisDef, peak, crossing)
+        loc = (max(lower, axisDef), peak, crossing)
         scalar = 1
 
         # The part before the crossing point.
@@ -175,7 +177,7 @@ def _solve(tent, axisLimit, negative=False):
         #              axisDef      axisMax
         #
         newUpper = peak + (1 - gain) * (upper - peak)
-        assert axisMax <= newUpper  # Because outGain >= gain
+        assert axisMax <= newUpper  # Because outGain > gain
         if newUpper <= axisDef + (axisMax - axisDef) * 2:
             upper = newUpper
             if not negative and axisDef + (axisMax - axisDef) * MAX_F2DOT14 < upper:

Tests/varLib/instancer/solver_test.py

@@ -276,6 +276,14 @@ class RebaseTentTest(object):
                     (1.0, (0.5, 1.0, 1.0)),
                 ],
             ),
+            # https://github.com/fonttools/fonttools/issues/3291
+            pytest.param(
+                (0.6, 0.7, 0.8),
+                (-1, 0.2, +1, 1, 1),
+                [
+                    (1.0, (0.5, 0.625, 0.75)),
+                ],
+            ),
         ],
     )
     def test_rebaseTent(self, tent, axisRange, expected):