TimelineQueueButtonHeader: display only when scrolled down

app/soapbox/components/timeline_queue_button_header.js

@@ -1,8 +1,9 @@
 import React from 'react';
 import PropTypes from 'prop-types';
 import { injectIntl } from 'react-intl';
+import { throttle } from 'lodash';
 import classNames from 'classnames';
-import SvgIcon from 'soapbox/components/svg_icon';
+import Icon from 'soapbox/components/icon';
 
 export default @injectIntl
 class TimelineQueueButtonHeader extends React.PureComponent {
@@ -11,25 +12,68 @@ class TimelineQueueButtonHeader extends React.PureComponent {
     onClick: PropTypes.func.isRequired,
     count: PropTypes.number,
     message: PropTypes.object.isRequired,
+    threshold: PropTypes.number,
     intl: PropTypes.object.isRequired,
   };
 
   static defaultProps = {
     count: 0,
+    threshold: 400,
   };
 
+  state = {
+    scrolled: false,
+  }
+
+  componentDidMount() {
+    this.window = window;
+    this.documentElement = document.scrollingElement || document.documentElement;
+
+    this.attachScrollListener();
+  }
+
+  componentWillUnmount() {
+    this.detachScrollListener();
+  }
+
+  attachScrollListener() {
+    this.window.addEventListener('scroll', this.handleScroll);
+  }
+
+  detachScrollListener() {
+    this.window.removeEventListener('scroll', this.handleScroll);
+  }
+
+  handleScroll = throttle(() => {
+    const { scrollTop } = (document.scrollingElement || document.documentElement);
+    const { threshold } = this.props;
+
+    if (scrollTop > threshold) {
+      this.setState({ scrolled: true });
+    } else {
+      this.setState({ scrolled: false });
+    }
+  }, 150, { trailing: true });
+
   render() {
     const { count, message, onClick, intl } = this.props;
+    const { scrolled } = this.state;
+
+    const visible = count > 0 && scrolled;
 
     const classes = classNames('timeline-queue-header', {
-      'hidden': (count <= 0),
+      'hidden': !visible,
     });
 
     return (
       <div className={classes}>
         <a className='timeline-queue-header__btn' onClick={onClick}>
-          <SvgIcon src={require('@tabler/icons/icons/arrow-bar-to-up.svg')} />
-          {(count > 0) && intl.formatMessage(message, { count })}
+          <Icon src={require('@tabler/icons/icons/arrow-bar-to-up.svg')} />
+          {(count > 0) && (
+            <div className='timeline-queue-header__label'>
+              {intl.formatMessage(message, { count })}
+            </div>
+          )}
         </a>
       </div>
     );

app/styles/components/remote-timeline.scss

@@ -29,9 +29,11 @@
 }
 
 .pinned-hosts-picker {
-  margin-left: 10px;
+  padding: 10px 0 0 10px;
   display: inline-flex;
   flex-wrap: wrap;
+  background: var(--foreground-color);
+  border-bottom: 1px solid hsla(var(--primary-text-color_hsl), 0.2);
 
   .pinned-host {
     margin-right: 10px;

app/styles/components/timeline-queue-header.scss

@@ -1,35 +1,35 @@
 .timeline-queue-header {
   display: flex;
+  align-self: center;
   align-items: center;
   justify-content: space-evenly;
-  max-height: 30px;
-  position: sticky;
+  height: 30px;
+  position: fixed;
   top: 60px;
   margin: 0 auto;
-  margin-bottom: 8px;
   background-color: var(--brand-color);
   color: #fff;
-  border-bottom: 1px solid;
-  border-top: 1px solid;
-  border-color: var(--brand-color--faint);
   border-radius: 100px;
-  transition: max-height 150ms ease;
+  transition: 150ms ease;
   overflow: hidden;
-  opacity: 1;
-  left: 0;
-  right: 0;
   padding: 0 10px;
   z-index: 500;
 
+  .sub-navigation ~ & {
+    top: calc(60px + 41px);
+  }
+
   .svg-icon {
     margin-right: 5px;
   }
 
   &.hidden {
-    max-height: 0;
-    opacity: 0;
-    margin: 0;
-    border: 0;
+    transform: scaleY(0);
+    pointer-events: none;
+
+    .timeline-queue-header__label {
+      opacity: 0;
+    }
   }
 
   &__btn {
@@ -46,4 +46,8 @@
       height: 46px;
     }
   }
+
+  &__label {
+    transition: 150ms ease;
+  }
 }