From b843ffb4d33ed57fe83f7d4d6696b712da03d024 Mon Sep 17 00:00:00 2001
From: Jan Petykiewicz <jan@mpxd.net>
Date: Mon, 30 Mar 2026 21:11:00 -0700
Subject: [PATCH] [ILibraryView / Pattern] flatten() shouldn't drop ports-only
 patterns if flatten_ports=True

---
 masque/library.py           |  3 ++-
 masque/pattern.py           |  3 ++-
 masque/test/test_library.py | 20 +++++++++++++++++++-
 masque/test/test_pattern.py | 15 +++++++++++++++
 4 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/masque/library.py b/masque/library.py
index 8dc63a4..5b32260 100644
--- a/masque/library.py
+++ b/masque/library.py
@@ -303,7 +303,8 @@ class ILibraryView(Mapping[str, 'Pattern'], metaclass=ABCMeta):
                 target_pat = flattened[target]
                 if target_pat is None:
                     raise PatternError(f'Circular reference in {name} to {target}')
-                if target_pat.is_empty():        # avoid some extra allocations
+                ports_only = flatten_ports and bool(target_pat.ports)
+                if target_pat.is_empty() and not ports_only:        # avoid some extra allocations
                     continue
 
                 for ref in pat.refs[target]:
diff --git a/masque/pattern.py b/masque/pattern.py
index 46b564a..9014298 100644
--- a/masque/pattern.py
+++ b/masque/pattern.py
@@ -1077,7 +1077,8 @@ class Pattern(PortList, AnnotatableImpl, Mirrorable):
 
                 if target_pat is None:
                     raise PatternError(f'Circular reference in {name} to {target}')
-                if target_pat.is_empty():        # avoid some extra allocations
+                ports_only = flatten_ports and bool(target_pat.ports)
+                if target_pat.is_empty() and not ports_only:        # avoid some extra allocations
                     continue
 
                 for ref in refs:
diff --git a/masque/test/test_library.py b/masque/test/test_library.py
index 22ad42a..5b143fa 100644
--- a/masque/test/test_library.py
+++ b/masque/test/test_library.py
@@ -2,7 +2,9 @@ import pytest
 from typing import cast, TYPE_CHECKING
 from ..library import Library, LazyLibrary
 from ..pattern import Pattern
-from ..error import LibraryError
+from ..error import LibraryError, PatternError
+from ..ports import Port
+from ..repetition import Grid
 
 if TYPE_CHECKING:
     from ..shapes import Polygon
@@ -59,6 +61,22 @@ def test_library_flatten() -> None:
     assert tuple(assert_vertices[0]) == (10.0, 10.0)
 
 
+def test_library_flatten_preserves_ports_only_child() -> None:
+    lib = Library()
+    child = Pattern(ports={"P1": Port((1, 2), 0)})
+    lib["child"] = child
+
+    parent = Pattern()
+    parent.ref("child", offset=(10, 10))
+    lib["parent"] = parent
+
+    flat_parent = lib.flatten("parent", flatten_ports=True)["parent"]
+
+    assert set(flat_parent.ports) == {"P1"}
+    assert cast("Port", flat_parent.ports["P1"]).rotation == 0
+    assert tuple(flat_parent.ports["P1"].offset) == (11.0, 12.0)
+
+
 def test_lazy_library() -> None:
     lib = LazyLibrary()
     called = 0
diff --git a/masque/test/test_pattern.py b/masque/test/test_pattern.py
index f5da195..338bd39 100644
--- a/masque/test/test_pattern.py
+++ b/masque/test/test_pattern.py
@@ -1,12 +1,15 @@
+import pytest
 from typing import cast
 from numpy.testing import assert_equal, assert_allclose
 from numpy import pi
 
+from ..error import PatternError
 from ..pattern import Pattern
 from ..shapes import Polygon
 from ..ref import Ref
 from ..ports import Port
 from ..label import Label
+from ..repetition import Grid
 
 
 def test_pattern_init() -> None:
@@ -99,6 +102,18 @@ def test_pattern_get_bounds() -> None:
     assert_equal(bounds, [[-5, -5], [10, 10]])
 
 
+def test_pattern_flatten_preserves_ports_only_child() -> None:
+    child = Pattern(ports={"P1": Port((1, 2), 0)})
+
+    parent = Pattern()
+    parent.ref("child", offset=(10, 10))
+
+    parent.flatten({"child": child}, flatten_ports=True)
+
+    assert set(parent.ports) == {"P1"}
+    assert parent.ports["P1"].rotation == 0
+    assert tuple(parent.ports["P1"].offset) == (11.0, 12.0)
+
 def test_pattern_interface() -> None:
     source = Pattern()
     source.ports["A"] = Port((10, 20), 0, ptype="test")