Add default portal containerRef context

packages/portal/src/reach-portal.tsx

@@ -20,6 +20,9 @@ import { createPortal } from "react-dom";
 
 declare const __DEV__: boolean;
 
+const PortalContainerContext = React.createContext<React.RefObject<Node> | null>(null);
+const { Provider: PortalContainerContextProvider } = PortalContainerContext;
+
 /**
  * Portal
  *
@@ -34,6 +37,8 @@ const PortalImpl: React.FC<PortalProps> = ({
 	let portalNode = React.useRef<HTMLElement | null>(null);
 	let forceUpdate = useForceUpdate();
 
+	const contextContainerRef = React.useContext(PortalContainerContext);
+
 	if (__DEV__) {
 		// eslint-disable-next-line react-hooks/rules-of-hooks
 		React.useEffect(() => {
@@ -54,6 +59,23 @@ const PortalImpl: React.FC<PortalProps> = ({
 							"See https://reactjs.org/docs/forwarding-refs.html."
 					);
 				}
+			} else if (contextContainerRef != null) {
+				if (typeof contextContainerRef !== "object" || !("current" in contextContainerRef)) {
+					console.warn(
+						"@reach/portal: Invalid value passed to the `PortalContainerContext`" +
+							"provider. The portal will be appended to the document body, but if " +
+							"you want to attach it to another DOM node you must pass a valid " +
+							"React ref object to `containerRef`."
+					);
+				} else if (contextContainerRef.current == null) {
+					console.warn(
+						"@reach/portal: A ref was passed to the `PortalContainerContext`" +
+							"provider, but no DOM node was attached to it. Be sure to pass the " +
+							"ref to a DOM component.\n\nIf you are forwarding the ref from " +
+							"another component, be sure to use the React.forwardRef API. " +
+							"See https://reactjs.org/docs/forwarding-refs.html."
+					);
+				}
 			}
 		}, [containerRef]);
 	}
@@ -64,7 +86,7 @@ const PortalImpl: React.FC<PortalProps> = ({
 		// It's possible that the content of the portal has, itself, been portaled.
 		// In that case, it's important to append to the correct document element.
 		let ownerDocument = mountNode.current!.ownerDocument;
-		let body = containerRef?.current || ownerDocument.body;
+		let body = containerRef?.current || contextContainerRef?.current || ownerDocument.body;
 		portalNode.current = ownerDocument?.createElement(type)!;
 		body.appendChild(portalNode.current);
 		forceUpdate();
@@ -131,4 +153,4 @@ Portal.displayName = "Portal";
 // Exports
 
 export type { PortalProps };
-export { Portal };
+export { Portal, PortalContainerContextProvider };