Switching difficulty now forces the selection to be cleared.

source/funkin/ui/debug/charting/ChartEditorState.hx

@@ -1739,6 +1739,10 @@ class ChartEditorState extends UIState // UIState derives from MusicBeatState
     noteTooltipsDirty = true;
     notePreviewViewportBoundsDirty = true;
 
+    // Switching difficulties should automatically clear the selection.
+    currentNoteSelection = [];
+    currentEventSelection = [];
+
     switchToCurrentInstrumental();
     postLoadInstrumental();
 
@@ -1762,6 +1766,10 @@ class ChartEditorState extends UIState // UIState derives from MusicBeatState
     noteTooltipsDirty = true;
     notePreviewViewportBoundsDirty = true;
 
+    // Switching difficulties should automatically clear the selection.
+    currentNoteSelection = [];
+    currentEventSelection = [];
+
     return selectedDifficulty;
   }