void remove(ComponentPlaceholder oldPlaceholder) {

		ComponentProvider provider = oldPlaceholder.getProvider();
		ComponentPlaceholder currentPlaceholder = activePlaceholderMap.get(provider);
		if (currentPlaceholder == oldPlaceholder) {
			activePlaceholderMap.remove(provider);
		}

		unusedPlaceholders.remove(oldPlaceholder);
		manager.disposePlaceholder(oldPlaceholder, false);
	}
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