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packages/block-editor/src/components/inserter-draggable-blocks/index.js

@@ -59,6 +59,10 @@ const InserterDraggableBlocks = ( {
 				startDragging();
 				for ( const block of draggableBlocks ) {
 					const type = `wp-block:${ block.name }`;
+					// This will fill in the dataTransfer.types array so that
+					// the drop zone can check if the draggable is eligible.
+					// Unfortuantely, on drag start, we don't have access to the
+					// actual data, only the data keys/types.
 					event.dataTransfer.items.add( '', type );
 				}
 			} }