MAGETWO-95866 Add horizontal scroll if elements extend menu's width

Author: ptylek

app/design/adminhtml/Magento/backend/Magento_Backend/web/css/source/module/_menu.less

@@ -17,7 +17,7 @@
 
 @menu-logo__padding-bottom: 1.7rem;
 @menu-logo__outer-size: @menu-logo__padding-top + @menu-logo-img__height + @menu-logo__padding-bottom;
-@menu-logo__padding-top: 1.7rem; 
+@menu-logo__padding-top: 1.7rem;
 @menu-logo-img__height: 4.1rem;
 @menu-logo-img__width: 3.5rem;
 
@@ -37,6 +37,7 @@
 @submenu__padding-horizontal: 1.5rem;
 @submenu__padding-vertical: 2rem;
 @submenu__z-index: @menu__z-index - 2;
+@submenu__height: 720px;
 @submenu-column__width: 23.8rem;
 @submenu-column__width__l: 19.8rem;
 @submenu-title__color: @color-white;
@@ -252,7 +253,6 @@
             background-color: @submenu__background-color;
             box-shadow: 0 0 3px @color-black;
             left: 100%; // align all submenus with one Y axis line
-            min-height: ~'calc(@{menu-logo__outer-size} + 2rem + 100%)';
             padding: @submenu__padding-vertical 0 0;
             position: absolute;
             top: 0;
@@ -266,6 +266,13 @@
             .ie11 & {
                 height: 100%;
             }
+
+            > ul[role="menu"] {
+                max-width:  ~'calc(100vw - @{menu__width})';
+                min-height: @submenu__height;
+                overflow-y: hidden;
+                overflow-x: auto;
+            }
         }
 
         &._show {