fix: regression in indexing when adding elements to frame (#6904)

src/tests/helpers/api.ts

@@ -17,6 +17,7 @@ import path from "path";
 import { getMimeType } from "../../data/blob";
 import {
   newEmbeddableElement,
+  newFrameElement,
   newFreeDrawElement,
   newImageElement,
 } from "../../element/newElement";
@@ -24,6 +25,7 @@ import { Point } from "../../types";
 import { getSelectedElements } from "../../scene/selection";
 import { isLinearElementType } from "../../element/typeChecks";
 import { Mutable } from "../../utility-types";
+import { assertNever } from "../../utils";
 
 const readFile = util.promisify(fs.readFile);
 
@@ -244,6 +246,15 @@ export class API {
           scale: rest.scale || [1, 1],
         });
         break;
+      case "frame":
+        element = newFrameElement({ ...base, width, height });
+        break;
+      default:
+        assertNever(
+          type,
+          `API.createElement: unimplemented element type ${type}}`,
+        );
+        break;
     }
     if (element.type === "arrow") {
       element.startBinding = rest.startBinding ?? null;