Thanks for using Announcer plugin.
The content shifts down to not hide the page content. This is one of the features of the plugin 😉
Right now there is no option to stop doing this.
But I can suggest two solutions.
1) Set below CSS anywhere in theme settings. This will simply stop shifting the content. (remove style tag if the setting will insert it already)
height: 0 !important;
2) Open the announcement after few seconds of page load.