Solution

src/App.scss

@@ -1,3 +1,16 @@
+.tile {
+  &.is-ancestor {
+    display: flex;
+  }
+
+  &.is-parent {
+    display: flex;
+    flex-direction: column;
+    flex-grow: 1;
+    padding: 15px;
+  }
+}
+
 .Sidebar {
   overflow: hidden;
   opacity: 0;
@@ -21,3 +34,4 @@
 .message-body {
   white-space: pre-line;
 }
+

src/App.tsx

@@ -1,60 +1,134 @@
-import classNames from 'classnames';
-
 import 'bulma/css/bulma.css';
 import '@fortawesome/fontawesome-free/css/all.css';
 import './App.scss';
 
+import { useEffect, useState } from 'react';
+import { User } from './types/User';
 import { PostsList } from './components/PostsList';
-import { PostDetails } from './components/PostDetails';
 import { UserSelector } from './components/UserSelector';
 import { Loader } from './components/Loader';
+import { client } from './utils/fetchClient';
+import { Post } from './types/Post';
+import classNames from 'classnames';
+import { PostDetails } from './components/PostDetails';
 
-export const App = () => (
-  <main className="section">
-    <div className="container">
-      <div className="tile is-ancestor">
-        <div className="tile is-parent">
-          <div className="tile is-child box is-success">
-            <div className="block">
-              <UserSelector />
-            </div>
+export const App = () => {
+  const [users, setUsers] = useState<User[]>([]);
+  const [selectedUser, setSelectedUser] = useState<User | null>(null);
+  const [posts, setPosts] = useState<Post[]>([]);
+  const [isLoading, setIsLoading] = useState(false);
+  const [error, setError] = useState('');
+  const [selectedPost, setSelectedPost] = useState<Post | null>(null);
 
-            <div className="block" data-cy="MainContent">
-              <p data-cy="NoSelectedUser">No user selected</p>
+  const loadUsers = async () => {
+    setIsLoading(true);
+    setError('');
+    try {
+      const fetchedUsers = await client.get<User[]>('/users');
 
-              <Loader />
+      if (fetchedUsers.length === 0) {
+        throw new Error('No users found');
+      }
 
-              <div
-                className="notification is-danger"
-                data-cy="PostsLoadingError"
-              >
-                Something went wrong!
-              </div>
+      setUsers(fetchedUsers);
+    } catch (err) {
+      setError('Failed to load users');
+    } finally {
+      setIsLoading(false);
+    }
+  };
+
+  const loadUserPosts = async (userId: number) => {
+    setIsLoading(true);
+    setError('');
+    try {
+      const fetchedPosts = await client.get<Post[]>(`/posts?userId=${userId}`);
 
-              <div className="notification is-warning" data-cy="NoPostsYet">
-                No posts yet
+      setPosts(fetchedPosts);
+    } catch (err) {
+      setError('Failed to load posts');
+    } finally {
+      setIsLoading(false);
+    }
+  };
+
+  useEffect(() => {
+    if (selectedUser) {
+      loadUserPosts(selectedUser.id);
+      setSelectedPost(null);
+    }
+  }, [selectedUser]);
+
+  useEffect(() => {
+    loadUsers();
+  }, []);
+
+  return (
+    <main className="section">
+      <div className="container">
+        <div className="tile is-ancestor">
+          <div className="tile is-parent">
+            <div className="tile is-child box is-success">
+              <div className="block">
+                <UserSelector
+                  users={users}
+                  selectedUser={selectedUser}
+                  onSelectUser={setSelectedUser}
+                />
               </div>
 
-              <PostsList />
+              <div className="block" data-cy="MainContent">
+                {!selectedUser && posts.length === 0 && !isLoading && (
+                  <p data-cy="NoSelectedUser">No user selected</p>
+                )}
+                {isLoading && <Loader />}
+                {error && (
+                  <div
+                    data-cy="PostsLoadingError"
+                    className="notification is-danger"
+                  >
+                    {error}
+                  </div>
+                )}
+
+                {!isLoading && !!posts.length ? (
+                  <PostsList
+                    posts={posts}
+                    openedPost={selectedPost}
+                    onOpen={setSelectedPost}
+                  />
+                ) : (
+                  !isLoading &&
+                  !error &&
+                  posts.length === 0 &&
+                  selectedUser && (
+                    <div
+                      className="notification is-warning"
+                      data-cy="NoPostsYet"
+                    >
+                      No posts yet
+                    </div>
+                  )
+                )}
+              </div>
             </div>
           </div>
-        </div>
-
-        <div
-          data-cy="Sidebar"
-          className={classNames(
-            'tile',
-            'is-parent',
-            'is-8-desktop',
-            'Sidebar',
-            'Sidebar--open',
-          )}
-        >
-          <div className="tile is-child box is-success ">
-            <PostDetails />
+          <div
+            data-cy="Sidebar"
+            className={classNames(
+              'tile',
+              'is-parent',
+              'is-8-desktop',
+              'Sidebar',
+              { 'Sidebar--open': selectedPost },
+            )}
+          >
+            <div className="tile is-child box is-success">
+              {selectedPost && <PostDetails post={selectedPost} />}
+            </div>
           </div>
         </div>
       </div>
-    </div>
-  </main>
-);
+    </main>
+  );
+};