Hi David,
OK, let me try to explain my intention.
I need a single page app with a collapsible side panel and a full size map.
So I grabbed the Horizontal Panel widget as it has a collapsible side panel and a content panel.
The problem is, the UI settings in ExB does not allow me to set 100% height of any content inside the side panel or the content panel. Any widget you drop inside a left or right side of a Horizontal Panel is wrapped with a Row by default and the Row does not have any height settings (exb-horizontal-sidebar-row-style.png). You can only set a fixed height of a content inside the panels (exb-horizontal-sidebar-map-style.png), but it's not responsive.
You can see the sandbox app here: https://experience.arcgis.com/stemapp/79aad0f703774c0db9bae9cdb2e46b7a.
Thanks,
Matej