drop oncurve if it's impliable either before OR after rounding

ofter the rounding gives us an opportunity to make an oncurve impliable, for points that were not equidistant become so after round, so why not take it!

Lib/fontTools/ttLib/tables/_g_l_y_f.py

@@ -15,6 +15,7 @@ from fontTools.misc.fixedTools import (
     strToFixedToFloat as str2fl,
     otRound,
 )
+from fontTools.misc.vector import Vector
 from numbers import Number
 from . import DefaultTable
 from . import ttProgram
@@ -28,6 +29,7 @@ from fontTools.misc import xmlWriter
 from fontTools.misc.filenames import userNameToFileName
 from fontTools.misc.loggingTools import deprecateFunction
 from enum import IntFlag
+from functools import partial
 from types import SimpleNamespace
 from typing import Set
 
@@ -1537,6 +1539,19 @@ class Glyph(object):
         return result if result is NotImplemented else not result
 
 
+# Vector.__round__ uses the built-in (Banker's) `round` but we want
+# to use otRound below
+_roundv = partial(Vector.__round__, round=otRound)
+
+
+def _is_mid_point(p0: tuple, p1: tuple, p2: tuple) -> bool:
+    # True if p1 is in the middle of p0 and p2, either before or after rounding
+    p0 = Vector(p0)
+    p1 = Vector(p1)
+    p2 = Vector(p2)
+    return ((p0 + p2) * 0.5).isclose(p1) or _roundv(p0) + _roundv(p2) == _roundv(p1) * 2
+
+
 def dropImpliedOnCurvePoints(*interpolatable_glyphs: Glyph) -> Set[int]:
     """Drop impliable on-curve points from the (simple) glyph or glyphs.
 
@@ -1597,12 +1612,8 @@ def dropImpliedOnCurvePoints(*interpolatable_glyphs: Glyph) -> Set[int]:
                 nxt = i + 1 if i < last else start
                 if (flags[prv] & flagOnCurve) or flags[prv] != flags[nxt]:
                     continue
-                p0 = coords[prv]
-                p1 = coords[i]
-                p2 = coords[nxt]
-                if not math.isclose(p1[0] - p0[0], p2[0] - p1[0]) or not math.isclose(
-                    p1[1] - p0[1], p2[1] - p1[1]
-                ):
+                # we may drop the ith on-curve if halfway between previous/next off-curves
+                if not _is_mid_point(coords[prv], coords[i], coords[nxt]):
                     continue
 
                 may_drop.add(i)