add shim and test

packages/gatsby-script/src/__tests__/gatsby-script.tsx

@@ -16,10 +16,19 @@ const strategies: Array<ScriptStrategy> = [
   ScriptStrategy.idle,
 ]
 
+jest.mock(`../request-idle-callback-shim`, () => {
+  const originalModule = jest.requireActual(`../request-idle-callback-shim`)
+
+  return {
+    __esModule: true,
+    ...originalModule,
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    requestIdleCallback: jest.fn<any, any>(callback => callback()),
+  }
+})
+
 describe(`Script`, () => {
   beforeAll(() => {
-    // eslint-disable-next-line @typescript-eslint/no-explicit-any
-    global.requestIdleCallback = jest.fn<any, any>(callback => callback())
     // @ts-ignore Mock it out for now
     performance.getEntriesByName = jest.fn(() => [])
   })

packages/gatsby-script/src/request-idle-callback-shim.ts

@@ -0,0 +1,18 @@
+// https://developer.chrome.com/blog/using-requestidlecallback/#checking-for-requestidlecallback
+// https://github.com/vercel/next.js/blob/canary/packages/next/client/request-idle-callback.ts
+
+export const requestIdleCallback =
+  (typeof self !== `undefined` &&
+    self.requestIdleCallback &&
+    self.requestIdleCallback.bind(window)) ||
+  function (cb: IdleRequestCallback): number {
+    const start = Date.now()
+    return setTimeout(function () {
+      cb({
+        didTimeout: false,
+        timeRemaining: function () {
+          return Math.max(0, 50 - (Date.now() - start))
+        },
+      })
+    }, 1) as unknown as number
+  }