Fix problem with invalidate cache (#6520)

# References and relevant issues

closes #6518

# Description
Fix for #6518 with added tests

napari/layers/labels/_tests/test_labels.py

@@ -1567,6 +1567,15 @@ def on_event():
     assert event_emitted
 
 
+def test_invalidate_cache_when_change_slice():
+    layer = Labels(np.zeros((2, 4, 5), dtype=np.uint8))
+    assert layer._cached_labels is None
+    layer._setup_cache(layer._slice.image.raw)
+    assert layer._cached_labels is not None
+    layer._set_view_slice()
+    assert layer._cached_labels is None
+
+
 class TestLabels:
     @staticmethod
     def get_objects():

napari/layers/labels/labels.py

@@ -1007,8 +1007,10 @@ def _raw_to_displayed(
         mapped_labels : array
             Encoded colors mapped between 0 and 1 to be displayed.
         """
+
         if data_slice is None:
             data_slice = tuple(slice(0, size) for size in raw.shape)
+            self._cached_labels = None
         else:
             self._setup_cache(raw)