Steps to Reproduce

<zk>
	<listbox id="lb" multiple="true" checkmark="true">
		<listhead>
			<listheader label="name"/>
		</listhead>
		<listitem label="One" />
		<listitem label="Two" />
	</listbox>
	<button label="clear selection" onClick="lb.clearSelection()"/>
</zk>

Current Result

when toggling items "One" and "Two" individually the selectAll checkbox synchronizes automatically
also the "clear selection" button removes unchecks all 3 checkboxes as expected

Problems happen after checking selectAll once (using the checkbox in the listheader)
-> the automatic synchronization fails ...
e.g.:

1. when "One" is selected -> selecting "Two" doesn't update the selectAll checkbox anymore
2. when all 3 are checked, clicking "clear selection" doesn't update the selectAll checkbox

Expected Result

keep selectAll in sync with individual checkboxes

Workaround

use a ListModelList with a listitem-template (here an example using zscript, also possible in java)

<zk>
	<zscript><![CDATA[
		ListModelList items = new ListModelList();
		items.add("One");
		items.add("Two");
		items.setMultiple(true);
	]]></zscript>
	<listbox id="lb" model="${items}" checkmark="true">
		<listhead>
			<listheader label="name"/>
		</listhead>
		<template name="model">
			<listitem label="${each}"/>
		</template>
	</listbox>
	<button label="clear selection" onClick="items.clearSelection()"/>
</zk>