void resetPlaceholdersWithoutProviders() {
		for (ComponentPlaceholder placeholder : unusedPlaceholders) {
			if (!placeholder.hasProvider()) {
				placeholder.reset();
			}
		}
	}
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