frame bg: drop legacy color if flag missing

packages/excalidraw/data/restore.ts

@@ -509,9 +509,17 @@ export const restoreElement = (
         name: element.name ?? null,
       });
     case "frame":
+      const hasBackgroundEnabled = Object.prototype.hasOwnProperty.call(
+        element,
+        "backgroundEnabled",
+      );
+      const backgroundEnabled = (element as any).backgroundEnabled ?? false;
       return restoreElementWithProperties(element, {
         name: element.name ?? null,
-        backgroundEnabled: (element as any).backgroundEnabled ?? false,
+        backgroundEnabled,
+        ...(hasBackgroundEnabled
+          ? {}
+          : { backgroundColor: DEFAULT_ELEMENT_PROPS.backgroundColor }),
       });
 
     // Don't use default case so as to catch a missing an element type case.

packages/excalidraw/tests/data/restore.test.ts

@@ -95,7 +95,7 @@ describe("restoreElements", () => {
       null,
     )[0] as ExcalidrawFrameElement;
 
-    expect(restoredFrame.backgroundColor).toBe("#ffc9c9");
+    expect(restoredFrame.backgroundColor).toBe("transparent");
     expect(restoredFrame.backgroundEnabled).toBe(false);
   });