ComponentPlaceholder placeholderFreed(ComponentProvider provider) {
		ComponentPlaceholder removedPlaceholder = activePlaceholderMap.remove(provider);
		if (removedPlaceholder == null) {
			// don't think this can happen
			return null;
		}

		boolean keepAround = !containsAnyMatchingPlaceholder(removedPlaceholder);
		if (keepAround) {
			unusedPlaceholders.add(removedPlaceholder);
		}

		manager.disposePlaceholder(removedPlaceholder, keepAround);

		return removedPlaceholder;
	}
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