fix(fonts): one BDF face per thread

The shared face cache now hands every loader (FontManager, element_style,
DisplayManager, the harness) the same freetype.Face. FreeType does not allow
two threads to use one face at once, since load_char rewrites its glyph
slot, so key the cache by thread as well.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/common/bdf_font.py

@@ -101,9 +101,12 @@ def load_bdf_face(path: str, size_px: int) -> Tuple[Any, int]:
     font that was asked for at the size it can do. Callers that lay out by
     size need ``realised_px``, not the size they asked for.
 
-    Faces are cached and shared: a ``freetype.Face`` holds per-glyph state,
+    Faces are cached per thread. A ``freetype.Face`` holds per-glyph state
-    so treat one as owned by the thread that draws. Raises if the file can't
+    (``load_char`` rewrites its glyph slot), and FreeType does not allow two
-    be loaded at either size.
+    threads to use one face at once, so the display thread and a plugin's
+    update thread must never be handed the same object. Within a thread the
+    face is shared by every caller. Raises if the file can't be loaded at
+    either size.
     """
     if freetype is None:
         raise RuntimeError("freetype-py is not installed; BDF fonts need it")
@@ -111,7 +114,8 @@ def load_bdf_face(path: str, size_px: int) -> Tuple[Any, int]:
     abs_path = os.path.abspath(path)
     try:
         st = os.stat(abs_path)
-        key = (abs_path, size_px, st.st_mtime_ns, st.st_size)
+        key = (threading.get_ident(), abs_path, size_px,
+               st.st_mtime_ns, st.st_size)
     except OSError:
         key = None  # let freetype raise its own error below
 

test/test_bdf_font.py

@@ -263,6 +263,20 @@ def test_cache_is_bounded(monkeypatch):
     bdf_font.clear_face_cache()
 
 
+def test_each_thread_gets_its_own_face():
+    # FreeType forbids two threads using one face at once: load_char rewrites
+    # the face's glyph slot. Within a thread the face is shared.
+    import threading
+    path = str(FONTS_DIR / "5x7.bdf")
+    here, _ = load_bdf_face(path, 7)
+    assert load_bdf_face(path, 7)[0] is here
+    other = []
+    worker = threading.Thread(target=lambda: other.append(load_bdf_face(path, 7)[0]))
+    worker.start()
+    worker.join()
+    assert other and other[0] is not here
+
+
 def test_native_size_prefers_pixel_size_over_point_size():
     # 6x13.bdf is defined at 75dpi: SIZE says 12 (points), PIXEL_SIZE 13.
     assert read_bdf_native_size(str(FONTS_DIR / "6x13.bdf")) == 13