Add test for backwards compatibility

tests/integration/data/old_version_data.json

+{
+  "question_text": "Note: This is an example of data in the format used by prior versions of this block. Not in particular the old `size` data.",
+  "zones": [
+    {
+      "index": 1,
+      "width": 200,
+      "title": "Zone 1",
+      "height": 100,
+      "y": "200",
+      "x": "100",
+      "id": "zone-1"
+    },
+    {
+      "index": 2,
+      "width": 200,
+      "title": "Zone 2",
+      "height": 100,
+      "y": 0,
+      "x": 0,
+      "id": "zone-2"
+    }
+  ],
+  "items": [
+    {
+      "displayName": "1",
+      "feedback": {
+        "incorrect": "No 1",
+        "correct": "Yes 1"
+      },
+      "zone": "Zone 1",
+      "backgroundImage": "",
+      "id": 0,
+      "size": {
+        "width": "190px",
+        "height": "auto"
+      }
+    },
+    {
+      "displayName": "2",
+      "feedback": {
+        "incorrect": "No 2",
+        "correct": "Yes 2"
+      },
+      "zone": "Zone 2",
+      "backgroundImage": "",
+      "id": 1,
+      "size": {
+        "width": "190px",
+        "height": "100px"
+      },
+      "inputOptions": {
+        "value": 100,
+        "margin": 5
+      }
+    },
+    {
+      "displayName": "Pic",
+      "feedback": {
+        "incorrect": "",
+        "correct": ""
+      },
+      "zone": "Zone 1",
+      "backgroundImage": "data:image/svg+xml;base64,PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSI4MDAiIGhlaWdodD0iNjAwIiBzdHlsZT0iYmFja2dyb3VuZDogI2VlZjsiPjwvc3ZnPg==",
+      "id": 2,
+      "size": {
+        "width": "100px",
+        "height": "auto"
+      }
+    }
+  ],
+  "state": {
+    "items": {},
+    "finished": true
+  },
+  "feedback": {
+    "start": "Intro Feedback",
+    "finish": "Final Feedback"
+  },
+  "title": "Drag and Drop (Old-style data)"
+}
\ No newline at end of file

tests/integration/test_sizing.py

@@ -191,3 +191,38 @@ class SizingTests(InteractionTestBase, BaseIntegrationTest):
                     container_width=target_img_width,
                     expected_percent=expected_width_percent,
                 )
+
+
+class SizingBackwardsCompatibilityTests(InteractionTestBase, BaseIntegrationTest):
+    """
+    Test backwards compatibility with data generated in older versions of this block.
+
+    Older versions allowed authors to specify a fixed width and height for each draggable, in
+    pixels (new versions only have a configurable width, and it is a percent width).
+    """
+    PAGE_TITLE = 'Drag and Drop v2 Sizing Backwards Compatibility'
+    PAGE_ID = 'drag_and_drop_v2_sizing_backwards_compatibility'
+
+    @staticmethod
+    def _get_scenario_xml():
+        """
+        Set up the test scenario:
+            * One DndDv2 block using 'old_version_data.json'
+        """
+        dnd_block = "<drag-and-drop-v2 data='{data}'/>".format(
+            data=loader.load_unicode("data/old_version_data.json")
+        )
+        return "<vertical_demo>{}</vertical_demo>".format(dnd_block)
+
+    def test_draggable_sizes(self):
+        """ Test the fixed pixel widths set in old versions of the block """
+        self._expect_width_px(item_id=0, width_px=190, zone_id="Zone 1")
+        self._expect_width_px(item_id=1, width_px=190, zone_id="Zone 2")
+        self._expect_width_px(item_id=2, width_px=100, zone_id="Zone 1")
+
+    def _expect_width_px(self, item_id, width_px, zone_id):
+        item = self._get_unplaced_item_by_value(item_id)
+        self.assertEqual(item.size["width"], width_px)
+        self.place_item(item_id, zone_id)
+        item = self._get_placed_item_by_value(item_id)
+        self.assertEqual(item.size["width"], width_px)