add scroll indicator gradients to top and bottom of room sub list

src/components/structures/IndicatorScrollbar.js

@@ -0,0 +1,62 @@
+/*
+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.
+*/
+
+import React from "react";
+import AutoHideScrollbar from "./AutoHideScrollbar";
+
+export default class IndicatorScrollbar extends React.Component {
+    constructor(props) {
+        super(props);
+        this._collectScroller = this._collectScroller.bind(this);
+        this.checkOverflow = this.checkOverflow.bind(this);
+    }
+
+    _collectScroller(scroller) {
+        if (scroller && !this._scroller) {
+            this._scroller = scroller;
+            this._scroller.addEventListener("scroll", this.checkOverflow);
+            this.checkOverflow();
+        }
+    }
+
+    checkOverflow() {
+        const hasTopOverflow = this._scroller.scrollTop > 0;
+        const hasBottomOverflow = this._scroller.scrollHeight >
+            (this._scroller.scrollTop + this._scroller.clientHeight);
+        if (hasTopOverflow) {
+            this._scroller.classList.add("mx_IndicatorScrollbar_topOverflow");
+        } else {
+            this._scroller.classList.remove("mx_IndicatorScrollbar_topOverflow");
+        }
+        if (hasBottomOverflow) {
+            this._scroller.classList.add("mx_IndicatorScrollbar_bottomOverflow");
+        } else {
+            this._scroller.classList.remove("mx_IndicatorScrollbar_bottomOverflow");
+        }
+    }
+
+    componentWillUnmount() {
+        if (this._scroller) {
+            this._scroller.removeEventListener("scroll", this.checkOverflow);
+        }
+    }
+
+    render() {
+        return (<AutoHideScrollbar wrappedRef={this._collectScroller} {... this.props}>
+            { this.props.children }
+        </AutoHideScrollbar>);
+    }
+}