I looked at your code, and the issue is that <span id="sidebar" drawer> and <span id="content" main> are not visible to Errai UI, because of how the <paper-drawer-panel> renders these elements.
If you look into how the paper-drawer-panel works, you'll see that it renders your elements as a <div id="drawer"> and <div id="main">. Therefore in this case, the workaround would be to change your data-fields to "drawer" and "main" instead of "sidebar" and "content" in both the template and the Java class, since these ids are enforced by the paper-drawer-panel. But this is a very specific workaround, and it may be better to rearrange the templates to avoid accidentally putting Errai UI data-fields into the shadow DOM of a web component, where they cannot be accessed.
Hope this helps,
Completely overlooked that the paper-drawer-panel overwrites my id.
Thanks for clarifying,