void removeAll() {
		// copy the set to prevent concurrent modifications
		Set<Entry<ComponentProvider, ComponentPlaceholder>> entries =
			new HashSet<>(
				activePlaceholderMap.entrySet());

		for (Entry<ComponentProvider, ComponentPlaceholder> entry : entries) {
			ComponentPlaceholder placeholder = entry.getValue();
			manager.disposePlaceholder(placeholder, false);
		}
		activePlaceholderMap.clear();

		for (ComponentPlaceholder placeholder : unusedPlaceholders) {
			manager.disposePlaceholder(placeholder, false);
		}
		unusedPlaceholders.clear();
	}
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