Login and Registration: Populate username after password reset.

Accessibility: to meet WCAG 2.2/3.3.7: Redundant entry, the username should be auto-populated when a user performs a password reset.

There is an existing cookie set that contains this information, but was deleted before displaying the login form.

Move cookie deletion to occur after displaying login form and use to set `$user_login`. 

Props estelaris, alh0319, sabernhardt, oglekler, peterwilsoncc, rcreators, rishavdutta, chaion07, stoyangeorgiev, rinkalpagdar, pratiklondhe, lukasfritzedev, ferdoused, audrasjb, westonruter, joedolson.
Fixes #60726.
Built from https://develop.svn.wordpress.org/trunk@61610


git-svn-id: http://core.svn.wordpress.org/trunk@60921 1a063a9b-81f0-0310-95a4-ce76da25c4cd

Author: joedolson

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '7.0-alpha-61609';
+$wp_version = '7.0-alpha-61610';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.

wp-login.php

@@ -1000,7 +1000,6 @@ function wp_login_viewport_meta() {
 
 		if ( ( ! $errors->has_errors() ) && isset( $_POST['pass1'] ) && ! empty( $_POST['pass1'] ) ) {
 			reset_password( $user, $_POST['pass1'] );
-			setcookie( $rp_cookie, ' ', time() - YEAR_IN_SECONDS, $rp_path, COOKIE_DOMAIN, is_ssl(), true );
 			login_header(
 				__( 'Password Reset' ),
 				wp_get_admin_notice(
@@ -1487,6 +1486,14 @@ function wp_login_viewport_meta() {
 			wp_clear_auth_cookie();
 		}
 
+		// Obtain user from password reset cookie flow before clearing the cookie.
+		$rp_cookie = 'wp-resetpass-' . COOKIEHASH;
+		if ( isset( $_COOKIE[ $rp_cookie ] ) && is_string( $_COOKIE[ $rp_cookie ] ) ) {
+			$user_login      = sanitize_user( strtok( wp_unslash( $_COOKIE[ $rp_cookie ] ), ':' ) );
+			list( $rp_path ) = explode( '?', wp_unslash( $_SERVER['REQUEST_URI'] ) );
+			setcookie( $rp_cookie, ' ', time() - YEAR_IN_SECONDS, $rp_path, COOKIE_DOMAIN, is_ssl(), true );
+		}
+
 		login_header( __( 'Log In' ), '', $errors );
 
 		if ( isset( $_POST['log'] ) ) {