Display search popout at fixed screen position (#16463)

* Display search popout at fixed screen position

* Attach search popout to search box

app/javascript/mastodon/features/compose/components/search.js

@@ -140,8 +140,7 @@ class Search extends React.PureComponent {
           <Icon id='search' className={hasValue ? '' : 'active'} />
           <Icon id='times-circle' className={hasValue ? 'active' : ''} aria-label={intl.formatMessage(messages.placeholder)} />
         </div>
-
-        <Overlay show={expanded && !hasValue} placement='bottom' target={this}>
+        <Overlay show={expanded && !hasValue} placement='bottom' target={this} container={this}>
           <SearchPopout />
         </Overlay>
       </div>