Fix animation for compatibility with display: flex.

services/web/public/stylesheets/app/editor/feature-onboarding.less

@@ -8,13 +8,15 @@
 	bottom: 0;
 	left: 0;
 	display: flex;
-	justify-content: center;
-	align-items: center;
-	background-color: rgba(0, 0, 0, .85);
+ 	justify-content: center;
+ 	align-items: center;
+	background-image: linear-gradient(rgba(0, 0, 0, .85), rgba(0, 0, 0, .85));
+	background-repeat: no-repeat;
+	background-position-x: 0;
 	color: #FFF;
 	text-align: center;
 	z-index: 102;
-	transition: left ease-in-out @left-menu-animation-duration;
+	transition: background-position ease-in-out @left-menu-animation-duration;
 	overflow: auto;
 	&::before {
 		content: '';
@@ -29,7 +31,7 @@
 }
 
 .feat-onboard-step2 {
-	left: @left-menu-width;
+	background-position-x: @left-menu-width;
 	&::before {
 		content: '';
 		position: fixed;