Hi,
The documentation (https://developers.arcgis.com/calcite-design-system/components/panel/) on Panel doesn't label heightScale property as optional. Therefore I had this property on my panel. I wanted my panel to take 100% height, like the panels in the Mapviewer, but I couldn't get it right.
Until I dropped the heightScale property, and now it takes 100% available height.
After inspection of the Mapviewer HTML+CSS I noticed that Esri doesn't use the property on the Map viewer Panels either. So I'm assuming the property is indeed optional and the documentation is missing the Optional label. Am I right?