Make sure when on last block focus cannot enter the block. (#37965)

* Make sure when on last block focus cannot enter the block.

* Check if focus element is inside block and loop until it's not. Only for last block to keep it somewhat efficient.

* Make sure block is selected.

* Don't move focus forward until we're sure it's necessary.

* Reviewer suggested approach seems to work, commit.

packages/block-editor/src/components/block-tools/block-selection-button.js

@@ -194,7 +194,13 @@ function BlockSelectionButton( { clientId, rootClientId, blockElement } ) {
 				let nextTabbable;
 
 				if ( navigateDown ) {
-					nextTabbable = focus.tabbable.findNext( blockElement );
+					nextTabbable = blockElement;
+					do {
+						nextTabbable = focus.tabbable.findNext( nextTabbable );
+					} while (
+						nextTabbable &&
+						blockElement.contains( nextTabbable )
+					);
 
 					if ( ! nextTabbable ) {
 						nextTabbable =