fixed MY PARTS so mutating the result list has no effect

HISTORY.md

@@ -9,6 +9,7 @@
     * typing strings into the search-field again shows relevant blocks (regression from IME)
     * fixed project dialog's search-field behevior (regression from IME)
     * morphic collision detection off-by-1 fix, thanks, Dariusz!
+    * fixed MY PARTS so mutating the result list has no effect
     * fixed a typo in the OF-reporter's help screen, thanks, @jasonappah
     * enable costumes created in the vector editor to be stretchable in Firefox, thanks, @coproc
 * **Translation Updates:**
@@ -21,6 +22,7 @@
 * morphic: new "reactToInput" text-editing event
 * objects: fixed #2485 (find blocks and text-entry mode feature)
 * gui: fixed ProjectDialog's search field behavior for IME 
+* threads: fixed MY PARTS so mutating the result list has no effect
 
 ### 2019-10-11
 * objects, threads: accept a list of pixels in the SWITCH TO COSTUME block

snap.html

@@ -7,7 +7,7 @@
         <script type="text/javascript" src="src/morphic.js?version=2019-10-14"></script>
         <script type="text/javascript" src="src/widgets.js?version=2019-06-27"></script>
         <script type="text/javascript" src="src/blocks.js?version=2019-08-07"></script>
-        <script type="text/javascript" src="src/threads.js?version=2019-10-11"></script>
+        <script type="text/javascript" src="src/threads.js?version=2019-10-14"></script>
         <script type="text/javascript" src="src/objects.js?version=2019-10-14"></script>
         <script type="text/javascript" src="src/gui.js?version=2019-10-14"></script>
         <script type="text/javascript" src="src/paint.js?version=2019-06-27"></script>

src/threads.js

@@ -61,7 +61,7 @@ StageMorph, SpriteMorph, StagePrompterMorph, Note, modules, isString, copy,
 isNil, WatcherMorph, List, ListWatcherMorph, alert, console, TableMorph, Color,
 TableFrameMorph, ColorSlotMorph, isSnapObject, Map, newCanvas, Symbol*/
 
-modules.threads = '2019-October-11';
+modules.threads = '2019-October-14';
 
 var ThreadManager;
 var Process;
@@ -4293,8 +4293,9 @@ Process.prototype.reportGet = function (query) {
                         each !== thisObj;
                 })
             );
-        case 'parts':
-            return new List(thisObj.parts || []);
+        case 'parts': // shallow copy to disable side-effects
+            return new List((thisObj.parts || [])
+                .map(function (each) {return each; }));
         case 'anchor':
             return thisObj.anchor || '';
         case 'parent':