Fix logged-out web UI on smaller screens (#19263)

app/javascript/mastodon/features/ui/components/header.js

@@ -0,0 +1,53 @@
+import React from 'react';
+import Logo from 'mastodon/components/logo';
+import { Link } from 'react-router-dom';
+import { FormattedMessage } from 'react-intl';
+import { registrationsOpen, me } from 'mastodon/initial_state';
+import Avatar from 'mastodon/components/avatar';
+import Permalink from 'mastodon/components/permalink';
+import PropTypes from 'prop-types';
+import { connect } from 'react-redux';
+
+const Account = connect(state => ({
+  account: state.getIn(['accounts', me]),
+}))(({ account }) => (
+  <Permalink href={account.get('url')} to={`/@${account.get('acct')}`}>
+    <span style={{ display: 'none' }}>{account.get('acct')}</span>
+    <Avatar account={account} size={35} />
+  </Permalink>
+));
+
+export default class Header extends React.PureComponent {
+
+  static contextTypes = {
+    identity: PropTypes.object,
+  };
+
+  render () {
+    const { signedIn } = this.context.identity;
+
+    let content;
+
+    if (signedIn) {
+      content = <Account />;
+    } else {
+      content = (
+        <React.Fragment>
+          <a href='/auth/sign_in' className='button'><FormattedMessage id='sign_in_banner.sign_in' defaultMessage='Sign in' /></a>
+          <a href={registrationsOpen ? '/auth/sign_up' : 'https://joinmastodon.org/servers'} className='button button-tertiary'><FormattedMessage id='sign_in_banner.create_account' defaultMessage='Create account' /></a>
+        </React.Fragment>
+      );
+    }
+
+    return (
+      <div className='ui__header'>
+        <Link to='/' className='ui__header__logo'><Logo /></Link>
+
+        <div className='ui__header__links'>
+          {content}
+        </div>
+      </div>
+    );
+  }
+
+}