Export combineParameters and document in migrations

MIGRATION.md

@@ -9,6 +9,7 @@
     - [Rolling back](#rolling-back)
   - [New addon presets](#new-addon-presets)
   - [Removed Deprecated APIs](#removed-deprecated-apis)
+  - [New setStories event](#new-setstories-event)
   - [Client API changes](#client-api-changes)
     - [Removed Legacy Story APIs](#removed-legacy-story-apis)
     - [Can no longer add decorators/parameters after stories](#can-no-longer-add-decoratorsparameters-after-stories)
@@ -261,6 +262,31 @@ See the migration guides for further details:
 - [Unified docs preset](#unified-docs-preset)
 - [Addon centered decorator deprecated](#addon-centered-decorator-deprecated)
 
+### New setStories event
+
+The `setStories`/`SET_STORIES` event has changed and now denormalizes global and kind-level parameters. The new format of the event data is:
+
+```js
+{
+  globalParameters: { p: 'q' },
+  kindParameters: { kind: { p: 'q' } },
+  stories: /* as before but with only story-level parameters */
+}
+```
+
+If you want the full denormalized parameters for a story, you can do something like:
+
+```js
+import { combineParameters } from '@storybook/api';
+
+const story = data.stories[storyId];
+const parameters = combineParameters(
+  data.globalParameters,
+  data.kindParameters[story.kind],
+  story.parameters
+);
+```
+
 ### Client API changes
 
 #### Removed Legacy Story APIs

lib/api/src/index.tsx

@@ -9,6 +9,7 @@ import React, {
   useMemo,
   useRef,
 } from 'react';
+import { mergeWith } from 'lodash';
 
 import {
   STORY_CHANGED,
@@ -116,6 +117,15 @@ export interface Parameters {
   [key: string]: any;
 }
 
+// This is duplicated from @storybook/client-api for the reasons mentioned in lib-addons/types.js
+export const combineParameters = (...parameterSets: Parameters[]) =>
+  mergeWith({}, ...parameterSets, (objValue: any, srcValue: any) => {
+    // Treat arrays as scalars:
+    if (Array.isArray(srcValue)) return srcValue;
+
+    return undefined;
+  });
+
 export type ModuleFn = (m: ModuleArgs) => Module;
 
 interface Module {

lib/api/src/lib/stories.ts

@@ -1,9 +1,9 @@
 import deprecate from 'util-deprecate';
 import dedent from 'ts-dedent';
 import { sanitize, parseKind } from '@storybook/csf';
-import { mapValues, mergeWith } from 'lodash';
+import { mapValues } from 'lodash';
 
-import { StoryId, StoryKind, Args, Parameters } from '../index';
+import { StoryId, StoryKind, Args, Parameters, combineParameters } from '../index';
 import merge from './merge';
 import { Provider } from '../modules/provider';
 
@@ -140,15 +140,6 @@ const toGroup = (name: string) => ({
   id: toKey(name),
 });
 
-// This is duplicated from @storybook/client-api for the reasons mentioned in lib-addons/types.js
-const combineParameters = (...parameterSets: Parameters[]) =>
-  mergeWith({}, ...parameterSets, (objValue: any, srcValue: any) => {
-    // Treat arrays as scalars:
-    if (Array.isArray(srcValue)) return srcValue;
-
-    return undefined;
-  });
-
 export const denormalizeStoryParameters = ({
   globalParameters,
   kindParameters,