Bring in TabbedView nearly verbatim from prior work

Sourced from https://github.com/matrix-org/matrix-react-sdk/pull/1644 and related PRs.

src/components/structures/TabbedView.js

@@ -0,0 +1,165 @@
+/*
+Copyright 2017 Travis Ralston
+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 * as React from "react";
+import {_t, _td} from '../../languageHandler';
+import GeminiScrollbar from 'react-gemini-scrollbar';
+import PropTypes from "prop-types";
+//import scrollSnapPolyfill from 'css-scroll-snap-polyfill';
+
+const DEFAULT_EXIT_STRING = _td("Return to app");
+
+/**
+ * Represents a tab for the TabbedView
+ */
+export class Tab {
+    /**
+     * Creates a new tab
+     * @param {string} tabLabel The untranslated tab label
+     * @param {string} tabJsx The JSX for the tab container.
+     */
+    constructor(tabLabel, tabJsx) {
+        this.label = tabLabel;
+        this.body = tabJsx;
+    }
+}
+
+export class TabbedView extends React.Component {
+    constructor() {
+        super();
+
+        // This is used to track when the user has scrolled all the way up or down so we
+        // don't immediately start flipping between tabs.
+        this._reachedEndAt = 0;
+    }
+
+    getInitialState() {
+        return {
+            activeTabIndex: 0,
+        };
+    }
+
+    _getActiveTabIndex() {
+        return this.state ? this.state.activeTabIndex : 0;
+    }
+
+    /**
+     * Shows the given tab
+     * @param {Tab} tab the tab to show
+     * @private
+     */
+    _setActiveTab(tab) {
+        const idx = this.props.tabs.indexOf(tab);
+        if (idx !== -1) {
+            this.setState({activeTabIndex: idx});
+            this._reachedEndAt = 0; // reset scroll timer
+        }
+        else console.error("Could not find tab " + tab.label + " in tabs");
+    }
+
+    _nextTab() {
+        let targetIndex = this._getActiveTabIndex() + 1;
+        if (targetIndex < this.props.tabs.length) {
+            this.setState({activeTabIndex: targetIndex});
+            this._reachedEndAt = 0; // reset scroll timer
+        }
+    }
+
+    _previousTab() {
+        let targetIndex = this._getActiveTabIndex() - 1;
+        if (targetIndex >= 0) {
+            this.setState({activeTabIndex: targetIndex});
+            this._reachedEndAt = 0; // reset scroll timer
+        }
+    }
+
+    _getTabLabel(tab) {
+        let classes = "mx_TabbedView_tabLabel ";
+
+        const idx = this.props.tabs.indexOf(tab);
+        if (idx === this._getActiveTabIndex()) classes += "mx_TabbedView_tabLabel_active";
+
+        return (
+            <span className={classes} key={"tab_label_ " + tab.label}
+                  onClick={() => this._setActiveTab(tab)}>
+                {_t(tab.label)}
+            </span>
+        );
+    }
+
+    _getTabPanel(tab) {
+        return (
+            <div className="mx_TabbedView_tabPanel" key={"mx_tabpanel_" + tab.label}>
+                {tab.body}
+            </div>
+        );
+    }
+
+    componentDidUpdate() {
+        window.requestAnimationFrame(() => {
+            console.log("SCROLL SNAP POLYFILL: UPDATE");
+            //scrollSnapPolyfill();
+        });
+    }
+
+    componentDidMount() {
+        window.requestAnimationFrame(() => {
+            console.log("SCROLL SNAP POLYFILL: MOUNT");
+            //scrollSnapPolyfill();
+        });
+    }
+
+    render() {
+        const labels = [];
+        const tabs = [];
+
+        for (const tab of this.props.tabs) {
+            labels.push(this._getTabLabel(tab));
+            tabs.push(this._getTabPanel(tab));
+        }
+
+        const returnToApp = (
+            <span className="mx_TabbedView_tabLabel mx_TabbedView_exit" onClick={this.props.onExit}>
+                {_t(this.props.exitLabel || DEFAULT_EXIT_STRING)}
+            </span>
+        );
+
+        return (
+            <div className="mx_TabbedView">
+                <div className="mx_TabbedView_tabLabels">
+                    {returnToApp}
+                    {labels}
+                </div>
+                <div className="mx_TabbedView_tabPanels">
+                    {tabs}
+                </div>
+            </div>
+        );
+    }
+}
+
+TabbedView.PropTypes = {
+    // Called when the user clicks the "Exit" or "Return to app" button
+    onExit: PropTypes.func.isRequired,
+
+    // The untranslated label for the "Return to app" button.
+    // Default: "Return to app"
+    exitLabel: PropTypes.string,
+
+    // The tabs to show
+    tabs: PropTypes.arrayOf(Tab).isRequired,
+};