Given the bug found with the old approach, fix this by going and deleting all of the monitor blocks for each sprite and the global variables.

src/engine/runtime.js

@@ -1541,13 +1541,18 @@ class Runtime extends EventEmitter {
         return newThreads;
     }
 
+
     /**
      * Dispose all targets. Return to clean state.
      */
     dispose () {
         this.stopAll();
+        // Deleting each target's variable's monitors.
+        this.targets.forEach(target => {
+            if (target.isOriginal) target.deleteMonitors();
+        });
+
         this.targets.map(this.disposeTarget, this);
-        this.monitorBlocks = new Blocks(true);
         this._monitorState = OrderedMap({});
         // @todo clear out extensions? turboMode? etc.
 

src/engine/target.js

@@ -336,6 +336,26 @@ class Target extends EventEmitter {
         }
     }
 
+    /**
+     * Remove this target's monitors from the runtime state and remove the
+     * target-specific monitored blocks (e.g. local variables, global variables for the stage, x-position).
+     * NOTE: This does not delete any of the stage monitors like backdrop name.
+     */
+    deleteMonitors () {
+        this.runtime.requestRemoveMonitorByTargetId(this.id);
+        let targetSpecificMonitorBlockIds;
+        if (this.isStage) {
+            // This only deletes global variables and not other stage monitors like backdrop number.
+            targetSpecificMonitorBlockIds = Object.keys(this.variables);
+        } else {
+            targetSpecificMonitorBlockIds = Object.keys(this.runtime.monitorBlocks._blocks)
+                .filter(key => this.runtime.monitorBlocks._blocks[key].targetId === this.id);
+        }
+        for (const blockId of targetSpecificMonitorBlockIds) {
+            this.runtime.monitorBlocks.deleteBlock(blockId);
+        }
+    }
+
     /**
      * Create a clone of the variable with the given id from the dictionary of
      * this target's variables.

src/virtual-machine.js

@@ -936,12 +936,7 @@ class VirtualMachine extends EventEmitter {
             const restoreSprite = () => spritePromise.then(spriteBuffer => this.addSprite(spriteBuffer));
             // Remove monitors from the runtime state and remove the
             // target-specific monitored blocks (e.g. local variables)
-            this.runtime.requestRemoveMonitorByTargetId(targetId);
-            const targetSpecificMonitorBlockIds = Object.keys(this.runtime.monitorBlocks._blocks)
-                .filter(key => this.runtime.monitorBlocks._blocks[key].targetId === targetId);
-            for (const blockId of targetSpecificMonitorBlockIds) {
-                this.runtime.monitorBlocks.deleteBlock(blockId);
-            }
+            target.deleteMonitors();
             const currentEditingTarget = this.editingTarget;
             for (let i = 0; i < sprite.clones.length; i++) {
                 const clone = sprite.clones[i];