Ensure dragging block to sprite creates only one. Fixes #1087 (#1088)

src/gui.js

@@ -8757,9 +8757,9 @@ SpriteIconMorph.prototype.reactToDropOf = function (morph, hand) {
     morph.slideBackTo(hand.grabOrigin);
 };
 
-SpriteIconMorph.prototype.copyStack = function (block) {
+SpriteIconMorph.prototype.copyStack = async function (block) {
     var sprite = this.object,
-        dup = block.fullCopy(),
+        dup = block.id ? block.fullCopy() : block,
         // FIXME: This positioning can be problematic...
         y = Math.max(
             sprite.scripts.children.map(stack =>
@@ -8768,10 +8768,7 @@ SpriteIconMorph.prototype.copyStack = function (block) {
         ),
         position = new Point(this.object.scripts.left() + 20, y + 20);
 
-    dup.setPosition(position);
-    sprite.scripts.add(dup);
     dup.allComments().forEach(comment => comment.align(dup));
-    sprite.scripts.adjustBounds();
 
     // delete all local custom blocks (methods) that the receiver
     // doesn't understand
@@ -8785,7 +8782,8 @@ SpriteIconMorph.prototype.copyStack = function (block) {
     });
 
     dup.id = null;
-    SnapActions.addBlock(dup, sprite, position);
+    await SnapActions.addBlock(dup, sprite, position);
+    sprite.scripts.adjustBounds();
 };
 
 SpriteIconMorph.prototype.copyCostume = function (costume) {

test/blocks.spec.js

@@ -480,6 +480,25 @@ describe('blocks', function() {
         });
     });
 
+    describe('across sprites', function() {
+        beforeEach(() => driver.reset());
+
+        it('should add new block on sprite icon drop', async function() {
+            const {BlockMorph} = driver.globals();
+            const block = driver.palette().contents.children
+                .find(c => c instanceof BlockMorph);
+            const [spriteIcon] = driver.ide().corral.frame.contents.children;
+            driver.dragAndDrop(block, spriteIcon.center(), block.center());
+            await driver.actionsSettled();
+            const sprite = driver.ide().currentSprite;
+            assert.equal(
+                sprite.scripts.children.length,
+                1,
+                'Created multiple blocks in sprite'
+            );
+        });
+    });
+
     describe('moveBlock', function() {
         before(() => driver.reset());