create LazyRenderList component to trim list to visible part

src/components/views/elements/LazyRenderList.js

@@ -0,0 +1,33 @@
+/*
+Copyright 2019 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.
+*/
+
+const OVERFLOW_ITEMS = 2;
+
+export default function LazyRenderList(props) {
+    const {items, itemHeight, scrollTop, height, renderItem} = props;
+
+    const firstIdx = Math.max(0, Math.floor(scrollTop / itemHeight) - OVERFLOW_ITEMS);
+    const itemsAfterFirst = items.length - firstIdx;
+    const amount = Math.min(Math.ceil(height / itemHeight) + OVERFLOW_ITEMS, itemsAfterFirst);
+    const beforeSpace = firstIdx * itemHeight;
+    const itemsAfter = itemsAfterFirst - amount;
+    const afterSpace = itemsAfter * itemHeight;
+    const renderedItems = items.slice(firstIdx, firstIdx + amount);
+
+    return (<div style={{paddingTop: `${beforeSpace}px`, paddingBottom: `${afterSpace}px`}}>
+        { renderedItems.map(renderItem) }
+    </div>);
+}