Eliminate erratic conversation deletion marker placement

ConversationView.scroll was still shown after conversation deletion,
though empty, and was throwing off the placement of the marker.

securedrop_client/gui/widgets.py

@@ -3469,6 +3469,7 @@ def __init__(self):
         layout.addStretch()
         layout.addWidget(deletion_message)
         layout.addWidget(tear)
+        layout.addStretch()
 
         self.setLayout(layout)
 
@@ -3521,11 +3522,13 @@ def __init__(self, source_db_object: Source, controller: Controller):
 
     def update_deletion_markers(self, collection):
         if collection:
+            self.scroll.show()
             if collection[0].file_counter > 1:
                 self.deleted_conversation_marker.hide()
                 self.deleted_conversation_items_marker.show()
         elif self.source.interaction_count > 0:
             self.deleted_conversation_items_marker.hide()
+            self.scroll.hide()
             self.deleted_conversation_marker.show()
 
     def update_conversation(self, collection: list) -> None: