Index: src/wp-login.php
===================================================================
--- src/wp-login.php	(revision 43389)
+++ src/wp-login.php	(working copy)
@@ -1050,6 +1050,8 @@
 
 		if ( isset( $_POST['log'] ) ) {
 			$user_login = ( 'incorrect_password' == $errors->get_error_code() || 'empty_password' == $errors->get_error_code() ) ? esc_attr( wp_unslash( $_POST['log'] ) ) : '';
+		} elseif ( isset( $_GET['username'] ) ) {
+			$user_login = esc_attr( wp_unslash( $_GET['username'] ) );
 		}
 		$rememberme = ! empty( $_POST['rememberme'] );