fix: handle UTF-8-SIG in convert_to_utf8, add status_changed on error

- Extend the early-return guard to include 'utf8sig' so chardet's
  UTF-8-SIG label (BOM files) is treated as already UTF-8.
- Emit status_changed("Error converting file") in the except block,
  consistent with save_file and load_file error paths.
- Remove redundant inline TextDocument imports from TestConvertToUtf8
  (import already exists at module level).
- Upgrade test_no_op_when_no_document and test_no_op_when_already_utf8
  to assert the expected status_changed message is emitted.
- Add test_no_op_when_utf8_with_bom to cover the UTF-8-SIG guard.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: chrisdpurcell

src/viewmodels/main_viewmodel.py

@@ -179,8 +179,10 @@ def convert_to_utf8(self, current_text: str) -> None:
             self.status_changed.emit("No document loaded")
             return
         # Normalise: strip dashes and lowercase so 'UTF-8', 'utf-8', 'utf8' all match.
+        # UTF-8-SIG is the chardet name for UTF-8 with a BOM — treat it as already UTF-8
+        # to avoid needlessly re-saving BOM files.
         current_encoding = self._current_document.encoding.lower().replace("-", "")
-        if current_encoding in ("utf8",):
+        if current_encoding in {"utf8", "utf8sig"}:
             self.status_changed.emit("File is already UTF-8")
             return
         doc = TextDocument(
@@ -198,3 +200,4 @@ def convert_to_utf8(self, current_text: str) -> None:
             msg = f"Cannot convert file: {e}"
             logger.error(msg)
             self.error_occurred.emit(msg)
+            self.status_changed.emit("Error converting file")

tests/unit/test_main_viewmodel.py

@@ -153,7 +153,6 @@ def test_replace_all_uses_current_text_when_provided(self, vm, qtbot):
 class TestConvertToUtf8:
     def test_saves_file_with_utf8_encoding(self, vm, mock_file_svc, qtbot):
         """convert_to_utf8 must call save_file with encoding='utf-8'."""
-        from src.models.text_document import TextDocument
         mock_file_svc.open_file.return_value = TextDocument(
             filepath="/tmp/latin.txt", content="caf\u00e9", encoding="ISO-8859-1"
         )
@@ -165,7 +164,6 @@ def test_saves_file_with_utf8_encoding(self, vm, mock_file_svc, qtbot):
 
     def test_emits_encoding_detected_utf8(self, vm, mock_file_svc, qtbot):
         """convert_to_utf8 must emit encoding_detected('utf-8')."""
-        from src.models.text_document import TextDocument
         mock_file_svc.open_file.return_value = TextDocument(
             filepath="/tmp/latin.txt", content="caf\u00e9", encoding="ISO-8859-1"
         )
@@ -176,7 +174,6 @@ def test_emits_encoding_detected_utf8(self, vm, mock_file_svc, qtbot):
 
     def test_emits_file_saved(self, vm, mock_file_svc, qtbot):
         """convert_to_utf8 must emit file_saved after a successful save."""
-        from src.models.text_document import TextDocument
         mock_file_svc.open_file.return_value = TextDocument(
             filepath="/tmp/latin.txt", content="caf\u00e9", encoding="ISO-8859-1"
         )
@@ -187,21 +184,32 @@ def test_emits_file_saved(self, vm, mock_file_svc, qtbot):
 
     def test_no_op_when_already_utf8(self, vm, mock_file_svc, qtbot):
         """convert_to_utf8 must not save when encoding is already utf-8."""
-        from src.models.text_document import TextDocument
         mock_file_svc.open_file.return_value = TextDocument(
             filepath="/tmp/utf8.txt", content="hello", encoding="utf-8"
         )
         vm.load_file("/tmp/utf8.txt")
-        vm.convert_to_utf8("hello")
+        with qtbot.waitSignal(vm.status_changed, timeout=1000) as blocker:
+            vm.convert_to_utf8("hello")
         mock_file_svc.save_file.assert_not_called()
+        assert "already" in blocker.args[0].lower()
 
     def test_no_op_when_no_document(self, vm, qtbot):
-        """convert_to_utf8 with no loaded document must be silent."""
-        vm.convert_to_utf8("some text")  # must not raise
+        """convert_to_utf8 with no loaded document emits status_changed."""
+        with qtbot.waitSignal(vm.status_changed, timeout=1000) as blocker:
+            vm.convert_to_utf8("some text")
+        assert "no document" in blocker.args[0].lower()
+
+    def test_no_op_when_utf8_with_bom(self, vm, mock_file_svc, qtbot):
+        """UTF-8-SIG (BOM) files must be treated as already UTF-8."""
+        mock_file_svc.open_file.return_value = TextDocument(
+            filepath="/tmp/bom.txt", content="hello", encoding="UTF-8-SIG"
+        )
+        vm.load_file("/tmp/bom.txt")
+        vm.convert_to_utf8("hello")
+        mock_file_svc.save_file.assert_not_called()
 
     def test_emits_error_on_save_failure(self, vm, mock_file_svc, qtbot):
         """convert_to_utf8 must emit error_occurred when save_file raises."""
-        from src.models.text_document import TextDocument
         mock_file_svc.open_file.return_value = TextDocument(
             filepath="/tmp/latin.txt", content="caf\u00e9", encoding="ISO-8859-1"
         )