Popover: Fix issue with undefined getBoundingClientRect (#27445)

Fix an issue where the `Popover` component throws a runtime `TypeError: Cannot read property 'getBoundingClientRect' of undefined.`

This may happen in WordPress if a Popover is rendered in an iframe. It's been observed by rendering a `BlockList` component containing `RichText` in an iframe 😵

The issue is that `instanceof` checks fail across iframe boundaries, where `anchorRef instanceof window.Element` fails because `anchorRef` when it _is_ an instance of Element in the frame but not `window.Element`.

Instead of `instanceof` checks that fail across iframe boundaries, check for expected methods as the predicate.

packages/components/src/popover/index.js

@@ -71,11 +71,17 @@ function computeAnchorRect(
 			return;
 		}
 
-		if ( anchorRef instanceof window.Range ) {
+		// Duck-type to check if `anchorRef` is an instance of Range
+		// `anchorRef instanceof window.Range` checks will break across document boundaries
+		// such as in an iframe
+		if ( typeof anchorRef?.cloneRange === 'function' ) {
 			return getRectangleFromRange( anchorRef );
 		}
 
-		if ( anchorRef instanceof window.Element ) {
+		// Duck-type to check if `anchorRef` is an instance of Element
+		// `anchorRef instanceof window.Element` checks will break across document boundaries
+		// such as in an iframe
+		if ( typeof anchorRef?.getBoundingClientRect === 'function' ) {
 			const rect = anchorRef.getBoundingClientRect();
 
 			if ( shouldAnchorIncludePadding ) {