So there is #unified-extensions-panel, which contains #unified-extensions-view. But once you click on an extension, it actually opens #customizationui-widget-panel, which contains #customizationui-widget-multiview (also used for the Tab Manager Menu). And then, you have to set .webextension-popup-browser to use the full height and width of the popup so that extensions can take up the full space and allow scrolling.
I had to use the Browser Toolbox to discover this, as well as guess-and-check to see how to actually configure it for proper use of extensions like uBlock Origin.