fix(loading): Prevent isLoading from being true with fallbackData

When `fallbackData` is provided, `isLoading` should not be true initially.
Instead, `isValidating` should be true to indicate background revalidation.
This prevents a "flash of skeleton" UI when initial data is available.

Fixes vercel#3046

Author: Conqxeror

src/index/use-swr.ts

@@ -195,7 +195,7 @@ export const useSWRHandler = <Data = any, Error = any>(
 
       return {
         isValidating: true,
-        isLoading: true,
+        isLoading: isUndefined(fallback),
         ...snapshot
       }
     }
@@ -278,8 +278,8 @@ export const useSWRHandler = <Data = any, Error = any>(
 
   const returnedData = keepPreviousData
     ? isUndefined(cachedData)
-      // checking undefined to avoid null being fallback as well
-      ? isUndefined(laggyDataRef.current)
+      ? // checking undefined to avoid null being fallback as well
+        isUndefined(laggyDataRef.current)
         ? data
         : laggyDataRef.current
       : cachedData
@@ -390,7 +390,7 @@ export const useSWRHandler = <Data = any, Error = any>(
       const initialState: State<Data, Error> = { isValidating: true }
       // It is in the `isLoading` state, if and only if there is no cached data.
       // This bypasses fallback data and laggy data.
-      if (isUndefined(getCache().data)) {
+      if (isUndefined(getCache().data) && isUndefined(fallback)) {
         initialState.isLoading = true
       }
       try {