add JumpToBottomButtom component

res/css/_components.scss

@@ -98,6 +98,7 @@
 @import "./views/rooms/_AuxPanel.scss";
 @import "./views/rooms/_EntityTile.scss";
 @import "./views/rooms/_EventTile.scss";
+@import "./views/rooms/_JumpToBottomButton.scss";
 @import "./views/rooms/_LinkPreviewWidget.scss";
 @import "./views/rooms/_MemberDeviceInfo.scss";
 @import "./views/rooms/_MemberInfo.scss";

res/css/views/rooms/_JumpToBottomButton.scss

@@ -0,0 +1,69 @@
+/*
+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.
+*/
+
+@charset "utf-8";
+
+.mx_JumpToBottomButton {
+    z-index: 1000;
+    position: absolute;
+    // 12 because height is 50 but button is only 38 = 12+(50-38) = 24
+    bottom: 12px;
+    right: 24px;
+    width: 38px;
+    // give it a fixed height so the badge doesn't make
+    // it taller and pop upwards when visible
+    height: 50px;
+    text-align: center;
+}
+
+.mx_JumpToBottomButton_badge {
+    position: relative;
+    top: -12px;
+    border-radius: 16px;
+    font-weight: bold;
+    font-size: 12px;
+    line-height: 14px;
+    text-align: center;
+    // to be able to get it centered
+    // with text-align in parent
+    display: inline-block;
+    padding: 0 4px;
+    color: $secondary-accent-color;
+    background-color: $warning-color;
+}
+
+.mx_JumpToBottomButton_scrollDown {
+    position: relative;
+    height: 38px;
+    border-radius: 19px;
+    box-sizing: border-box;
+    background: $primary-bg-color;
+    border: 1.3px solid $roomtile-name-color;
+    cursor: pointer;
+}
+
+.mx_JumpToBottomButton_scrollDown:before {
+    content: "";
+    position: absolute;
+    top: 0;
+    bottom: 0;
+    left: 0;
+    right: 0;
+    mask: url('$(res)/img/icon-jump-to-bottom.svg');
+    mask-repeat: no-repeat;
+    mask-position: 9px 14px;
+    background: $roomtile-name-color;
+}

res/img/icon-jump-to-bottom.svg

@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   id="svg4"
+   width="18.666187"
+   height="8.7375822"
+   viewBox="0 0 18.666187 8.7375818"
+   version="1.1"
+   style="fill:none">
+  <metadata
+     id="metadata8">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title></dc:title>
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <defs
+     id="defs8" />
+  <path
+     id="path2-8-3"
+     style="fill:none;stroke:#000000;stroke-width:1.29999995;stroke-linecap:round;stroke-linejoin:round;stroke-miterlimit:4;stroke-dasharray:none;stroke-opacity:1"
+     d="M 17.909095,0.75599092 9.3330939,7.987602 0.75709259,0.75599092" />
+</svg>

src/components/views/rooms/JumpToBottomButton.js

@@ -0,0 +1,32 @@
+/*
+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.
+*/
+
+import { _t } from '../../../languageHandler';
+import AccessibleButton from '../elements/AccessibleButton';
+
+export default (props) => {
+    let badge;
+    if (props.numUnreadMessages) {
+        badge = <div className="mx_JumpToBottomButton_badge">{props.numUnreadMessages}</div>
+    }
+    return (<div className="mx_JumpToBottomButton">
+        <AccessibleButton className="mx_JumpToBottomButton_scrollDown"
+            title={_t("Scroll to bottom of page")}
+            onClick={props.onScrollToBottomClick}>
+        </AccessibleButton>
+        { badge }
+    </div>);
+};