[Forwardport 2.3] Trim username on customer account login page

app/code/Magento/Customer/view/frontend/templates/form/login.phtml

@@ -42,3 +42,13 @@
         </form>
     </div>
 </div>
+
+<script type="text/x-magento-init">
+    {
+        ".field.email": {
+            "Magento_Customer/js/trim-username": {
+                "formSelector": "form.form-login"
+            }
+        }
+    }
+</script>

app/code/Magento/Customer/view/frontend/web/js/trim-username.js

@@ -0,0 +1,65 @@
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+define([
+    'jquery'
+], function ($) {
+    'use strict';
+
+    $.widget('mage.trimUsername', {
+        options: {
+            cache: {},
+            formSelector: 'form',
+            emailSelector: 'input[type="email"]'
+        },
+
+        /**
+         * Widget initialization
+         * @private
+         */
+        _create: function () {
+            // We need to look outside the module for backward compatibility, since someone can already use the module.
+            // @todo Narrow this selector in 2.3 so it doesn't accidentally finds the email field from the
+            // newsletter email field or any other "email" field.
+            this.options.cache.email = $(this.options.formSelector).find(this.options.emailSelector);
+            this._bind();
+        },
+
+        /**
+         * Event binding, will monitor change, keyup and paste events.
+         * @private
+         */
+        _bind: function () {
+            if (this.options.cache.email.length) {
+                this._on(this.options.cache.email, {
+                    'change': this._trimUsername,
+                    'keyup': this._trimUsername,
+                    'paste': this._trimUsername
+                });
+            }
+        },
+
+        /**
+         * Trim username
+         * @private
+         */
+        _trimUsername: function () {
+            var username = this._getUsername().trim();
+
+            this.options.cache.email.val(username);
+        },
+
+        /**
+         * Get username value
+         * @returns {*}
+         * @private
+         */
+        _getUsername: function () {
+            return this.options.cache.email.val();
+        }
+    });
+
+    return $.mage.trimUsername;
+});