private boolean containsMatchingUnusedPlaceholder(ComponentPlaceholder matchee) {
		return getMatchingUnusedPlaceholder(matchee) != null;
	}
A class that tracks:
<ul>
 	<li>placeholders that are being used for a given provider</li>
 	<li>placeholders that are no longer being used, which are available for reuse </li>
</ul>
Ghidra/Framework/Docking/src/main/java/docking/PlaceholderSet.java