Fix invisible load more button (#4962)

* Fix behavior while the button is invisible
e.g. pointer cursor, couldn't open contextmenu
* Avoid rendering the button to remove blank space if no more items are available or no items are rendered

app/javascript/mastodon/components/load_more.js

@@ -17,7 +17,7 @@ export default class LoadMore extends React.PureComponent {
     const { visible } = this.props;
 
     return (
-      <button className='load-more' disabled={!visible} style={{ opacity: visible ? 1 : 0 }} onClick={this.props.onClick}>
+      <button className='load-more' disabled={!visible} style={{ visibility: visible ? 'visible' : 'hidden' }} onClick={this.props.onClick}>
         <FormattedMessage id='status.load_more' defaultMessage='Load more' />
       </button>
     );