--- a/mercurial/context.py	Fri Feb 07 16:40:22 2025 -0500
+++ b/mercurial/context.py	Mon Feb 10 18:14:44 2025 +0100
@@ -1222,14 +1222,16 @@
             # it is safe to use an unfiltered repository here because we are
             # walking ancestors only.
             cl = self._repo.unfiltered().changelog
-            if base.rev() is None:
+            # use self.rev(), not base.rev(), because if self is a merge  we should still
+            # consider linkrevs in the other branch as ancestors.
+            if self.rev() is None:
                 # wctx is not inclusive, but works because _ancestrycontext
                 # is used to test filelog revisions
                 ac = cl.ancestors(
-                    [p.rev() for p in base.parents()], inclusive=True
+                    [p.rev() for p in self.parents()], inclusive=True
                 )
             else:
-                ac = cl.ancestors([base.rev()], inclusive=True)
+                ac = cl.ancestors([self.rev()], inclusive=True)
             base._ancestrycontext = ac
 
         return dagop.annotate(

--- a/tests/test-annotate.t	Fri Feb 07 16:40:22 2025 -0500
+++ b/tests/test-annotate.t	Mon Feb 10 18:14:44 2025 +0100
@@ -1218,7 +1218,6 @@
   $ cd ..
 
 Annotate should use the starting revision (-r) as base for ancestor checks.
-TODO: Fix Python, which calls introfilectx() first and uses that for ancestor checks.
 
   $ hg init repo-base
   $ cd repo-base
@@ -1258,8 +1257,7 @@
 from 4 (starting revision), not from 3 (most recent change to the file).
   $ hg annotate file
   0: A
-  2: B (no-rhg known-bad-output !)
-  1: B (rhg !)
+  1: B
   3: C
   $ echo D >> file
   $ hg commit -m "another change"