AutoHideScrollbar component

shows scrollbar on hover when needed
has fallback to overflow: overlay
and fiddling with padding to make content not jump on hover.

res/css/structures/_AutoHideScrollbar.scss

@@ -0,0 +1,56 @@
+/*
+Copyright 2018 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/*
+1. for browsers that support native overlay auto-hiding scrollbars
+*/
+.mx_AutoHideScrollbar {
+    overflow-y: auto;
+    -ms-overflow-style: -ms-autohiding-scrollbar;
+}
+/*
+2. webkit also supports overflow:overlay where the scrollbars don't take any space
+in the layout but they don't autohide, so do that only on hover
+*/
+body.mx_scrollbar_overlay_noautohide .mx_AutoHideScrollbar {
+    overflow-y: hidden;
+}
+
+body.mx_scrollbar_overlay_noautohide .mx_AutoHideScrollbar:hover {
+    overflow-y: overlay;
+}
+/*
+3. as a last fallback, compensate for the scrollbar taking up space in the layout
+by playing with the paddings. the default below will add a right padding
+of the scrollbar width and clear that on hover.
+this won't work well on classes that also need to set their padding,
+so this needs to be overriden and adjust the padding with calc like so:
+```
+body.mx_scrollbar_nooverlay .componentClass.mx_AutoHideScrollbar_overflow:hover {
+    padding-right: calc(15px - var(--scrollbar-width)) !important;
+}
+```
+*/
+body.mx_scrollbar_nooverlay .mx_AutoHideScrollbar {
+    box-sizing: border-box;
+    overflow-y: hidden;
+    padding-right: var(--scrollbar-width);
+}
+
+body.mx_scrollbar_nooverlay .mx_AutoHideScrollbar:hover {
+    overflow-y: auto;
+    padding-right: 0;
+}