Index: src/wp-admin/css/common.css
===================================================================
--- src/wp-admin/css/common.css	(revision 48276)
+++ src/wp-admin/css/common.css	(working copy)
@@ -1086,8 +1086,22 @@
 	margin: 0;
 }

+/* Use flexbox only on the plugins install page. The children `filter-links` and search form will become flex items. */
+.plugin-install-php .wp-filter {
+	display: flex;
+	flex-wrap: wrap;
+	justify-content: space-between;
+}
+
 .wp-filter .search-form.search-plugins {
+	margin-top: 0;
+}
+
+.wp-filter .search-form.search-plugins select,
+.wp-filter .search-form.search-plugins .wp-filter-search {
 	display: inline-block;
+	margin-top: 10px;
+	vertical-align: top;
 }

 .wp-filter .button.drawer-toggle {
@@ -3968,6 +3982,11 @@
 	.wp-filter .search-form input[type="search"] {
 		font-size: 1rem;
 	}
+
+	.wp-filter .search-form.search-plugins {
+		/* This element is a flex item. */
+		min-width: 100%;
+	}
 }

 /* Smartphone */
Index: src/wp-admin/css/list-tables.css
===================================================================
--- src/wp-admin/css/list-tables.css	(revision 48276)
+++ src/wp-admin/css/list-tables.css	(working copy)
@@ -2094,6 +2094,16 @@
 		line-height: 1.5;
 	}

+	.plugins tbody th.check-column {
+		padding: 8px 0 0 5px;
+	}
+
+	.plugins thead td.check-column,
+	.plugins tfoot td.check-column,
+	.plugins .inactive th.check-column {
+		padding-left: 9px;
+	}
+
 	/* Add New plugins page */
 	table.plugin-install .column-name,
 	table.plugin-install .column-version,
Index: src/wp-admin/includes/plugin-install.php
===================================================================
--- src/wp-admin/includes/plugin-install.php	(revision 48276)
+++ src/wp-admin/includes/plugin-install.php	(working copy)
@@ -334,9 +334,8 @@
 			<option value="author"<?php selected( 'author', $type ); ?>><?php _e( 'Author' ); ?></option>
 			<option value="tag"<?php selected( 'tag', $type ); ?>><?php _ex( 'Tag', 'Plugin Installer' ); ?></option>
 		</select>
-		<label><span class="screen-reader-text"><?php _e( 'Search Plugins' ); ?></span>
-			<input type="search" name="s" value="<?php echo esc_attr( $term ); ?>" class="wp-filter-search" placeholder="<?php esc_attr_e( 'Search plugins...' ); ?>" />
-		</label>
+		<label class="screen-reader-text" for="search-plugins"><?php _e( 'Search Plugins' ); ?></label>
+		<input type="search" name="s" id="search-plugins" value="<?php echo esc_attr( $term ); ?>" class="wp-filter-search" placeholder="<?php esc_attr_e( 'Search plugins...' ); ?>" />
 		<?php submit_button( __( 'Search Plugins' ), 'hide-if-js', false, false, array( 'id' => 'search-submit' ) ); ?>
 	</form>
 	<?php