diff --git a/HISTORY.md b/HISTORY.md
index e42a0fc4..318cb2ab 100755
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -6,8 +6,9 @@
     * accept a list of pixels in the SWITCH TO COSTUME block
     * URLs in dialog boxes are now clickable and can open new browser tabs, thanks, Brian Broll! 
 * **Notable Changes:**
-* **Notable Fixes:**
     * made "i" upvar inside FOR loop's C-Shape slot mutable by user script
+    * prevent switching to another sprite if a block editor is open (so local blocks of different sprites don't mix)
+* **Notable Fixes:**
     * 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!
@@ -20,6 +21,9 @@
     * Galician, thanks, Bernat
     * Turkish, thanks, Turgut!
 
+### 2019-10-15
+* gui: prevent switching to another sprite if a block editor is open (so local blocks of different sprites don't mix)
+
 ### 2019-10-14
 * morphic: new "reactToInput" text-editing event
 * objects: fixed #2485 (find blocks and text-entry mode feature)
diff --git a/snap.html b/snap.html
index 1f7b6961..dc8d068a 100755
--- a/snap.html
+++ b/snap.html
@@ -9,7 +9,7 @@
         <script type="text/javascript" src="src/blocks.js?version=2019-08-07"></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/gui.js?version=2019-10-15"></script>
         <script type="text/javascript" src="src/paint.js?version=2019-06-27"></script>
         <script type="text/javascript" src="src/lists.js?version=2019-07-01"></script>
         <script type="text/javascript" src="src/byob.js?version=2019-07-24"></script>
diff --git a/src/gui.js b/src/gui.js
index 4959bf05..15acf016 100644
--- a/src/gui.js
+++ b/src/gui.js
@@ -60,7 +60,7 @@
 
 /*global modules, Morph, SpriteMorph, SyntaxElementMorph, Color, Cloud,
 ListWatcherMorph, TextMorph, newCanvas, useBlurredShadows, VariableFrame,
-StringMorph, Point, MenuMorph, morphicVersion, DialogBoxMorph,normalizeCanvas,
+StringMorph, Point, MenuMorph, morphicVersion, DialogBoxMorph, normalizeCanvas,
 ToggleButtonMorph, contains, ScrollFrameMorph, StageMorph, PushButtonMorph, sb,
 InputFieldMorph, FrameMorph, Process, nop, SnapSerializer, ListMorph, detect,
 AlignmentMorph, TabMorph, Costume, MorphicPreferences, Sound, BlockMorph,
@@ -71,11 +71,12 @@ CommentMorph, CommandBlockMorph, BooleanSlotMorph, RingReporterSlotMorph,
 BlockLabelPlaceHolderMorph, Audio, SpeechBubbleMorph, ScriptFocusMorph,
 XML_Element, WatcherMorph, BlockRemovalDialogMorph, saveAs, TableMorph,
 isSnapObject, isRetinaEnabled, disableRetinaSupport, enableRetinaSupport,
-isRetinaSupported, SliderMorph, Animation, BoxMorph, MediaRecorder*/
+isRetinaSupported, SliderMorph, Animation, BoxMorph, MediaRecorder,
+BlockEditorMorph, BlockDialogMorph*/
 
 // Global stuff ////////////////////////////////////////////////////////
 
-modules.gui = '2019-October-14';
+modules.gui = '2019-October-15';
 
 // Declarations
 
@@ -2151,6 +2152,19 @@ IDE_Morph.prototype.stopAllScripts = function () {
 };
 
 IDE_Morph.prototype.selectSprite = function (sprite) {
+    // prevent switching to another sprite if a block editor is open
+    // so local blocks of different sprites don't mix
+    if (
+        detect(
+            this.world().children,
+            function (morph) {
+                return morph instanceof BlockEditorMorph ||
+                    morph instanceof BlockDialogMorph;
+            }
+        )
+    ) {
+        return;
+    }
     if (this.currentSprite && this.currentSprite.scripts.focus) {
         this.currentSprite.scripts.focus.stopEditing();
     }