FIX: Cope with Matrix entry in field annotations (#2736)

Closes #2731

pypdf/_writer.py

@@ -906,6 +906,10 @@ def _update_field_annotation(
                 "/Length": 0,
             }
         )
+        if AA.AP in anno:
+            for k, v in cast(DictionaryObject, anno[AA.AP]).get("/N", {}).items():
+                if k not in {"/BBox", "/Length", "/Subtype", "/Type", "/Filter"}:
+                    dct[k] = v
 
         # Update Resources with font information if necessary
         if font_res is not None:

tests/test_writer.py

@@ -2286,3 +2286,17 @@ def test_field_box_upside_down():
     box = writer.pages[0]["/Annots"][13].get_object()["/AP"]["/N"]["/BBox"]
     assert box[2] > 0
     assert box[3] > 0
+
+
+@pytest.mark.enable_socket()
+def test_matrix_entry_in_field_annots():
+    """Cf #2731"""
+    url = "https://github.com/user-attachments/files/16036514/template.pdf"
+    name = "iss2731.pdf"
+    writer = PdfWriter(BytesIO(get_data_from_url(url, name=name)))
+    writer.update_page_form_field_values(
+        writer.pages[0],
+        {"Stellenbezeichnung_1": "some filled in text"},
+        auto_regenerate=False,
+    )
+    assert "/Matrix" in writer.pages[0]["/Annots"][5].get_object()["/AP"]["/N"]