some fixes

python_app/gui/controllers/app_window_preprocess_mixin.py

@@ -142,11 +142,19 @@ class AppWindowPreprocessMixin:
             )
             self._selected_preprocess_sets["s21_reference"] = TMP_REFERENCE_SET_NAME
             self._selected_preprocess_radar_key = radar_key
-            self._processor_run_signature = None
+            # Do NOT restart data_processor for a reference change: it does not consume
+            # the S21 reference (the data_preprocessor does), and restarting it would tear
+            # down the locator server it hosts and drop every connected client. The
+            # preprocessor reloads the new reference when acquisition restarts below; the
+            # processor keeps running. We only re-baseline the on-screen display history.
             self._history_run_signature = None
-            if self._supervisor.is_processor_running():
-                self._stop_all_processes()
             self._reset_runtime_history()
+            # Tell the still-running data_processor to drop its accumulated background/
+            # history so the new reference takes effect cleanly (no old/new-reference
+            # blend) without restarting the process. Reuses the live-config clear_all
+            # command, applied on the processor's next poll.
+            if self._supervisor.is_processor_running():
+                self._write_live_processing_config(history_command="clear_all", bump_history_seq=True)
             self._refresh_preprocess_summary_labels()
             if collection.traces:
                 self._draw_single_trace(