Sidebar categories list inside Admin Posts

function.php

add_action( 'admin_notices', 'add_html_and_category_filter' );

function add_html_and_category_filter() {
	$current_screen = get_current_screen();
	
	if ( $current_screen->id === 'edit-post' && $current_screen->post_type === 'post' ) {
		$categories = get_categories();

		$current_category_id = isset( $_GET['cat'] ) ? intval( $_GET['cat'] ) : 0;

		ob_start();
		?>
		<div id="category-filter-list">
			<h2><?php esc_html_e( 'Categories:' ); ?></h2>
			<ul>
				<li <?php if ( 0 === $current_category_id ) : ?>class="active"<?php endif; ?>>
					<a href="<?php echo esc_url( admin_url( 'edit.php?post_status=all&post_type=post' ) ); ?>"><?php esc_html_e( 'All Posts' ); ?></a>
				</li>
				<?php foreach ( $categories as $category ) : ?>
					<?php
					$category_link = esc_url( get_category_link( $category->cat_ID ) );
					$category_id   = $category->cat_ID;
					$category_count = $category->count;
					?>
					<li <?php if ( $category_id === $current_category_id ) : ?>class="active"<?php endif; ?>>
						<a href="<?php echo esc_url( admin_url( 'edit.php?s&post_status=all&post_type=post&action=-1&m=0&cat=' . $category_id . '&filter_action=Filter&paged=1&action2=-1' ) ); ?>"><?php echo esc_html( $category->name ); ?> (<?php echo esc_html( $category_count ); ?>)</a>
					</li>
				<?php endforeach; ?>
			</ul>
		</div>

		<style>
			#category-filter-list {
				position: absolute;
                padding-top: 10px;
				left: 0;
				width: 200px;
				height: 100%;
				z-index: 100;
			}
			#category-filter-list a {
				display: block;
				padding: 8px 0;
				font-size: 14px;
				border-bottom: 1px solid lightgray;
				text-decoration: none;
			}
			#category-filter-list li.active a {
				font-weight: bold;
			}

            .wrap{
                padding-left: 220px;
            }
		</style>
		<?php
		$output = ob_get_clean();
		
		echo $output;
	}
}

Here is the preview of the result

Updated code:

• Changed title to "Categories:"
• Removed description text
• Fixed overlap when user open "Screen options"
• Added active class selected category (Styled as bold)
• Added numbed of posts for category
• Added all categories like reset filter