SelectAll
configurationWhen set to true, renders a sticky "Select all" header at the top of the dropdown list. Clicking it selects all currently rendered items. When checkboxes is also enabled, the header shows a checkbox that reflects the aggregate selection state: unchecked (none selected), indeterminate (some selected), or checked (all selected).
The built-in click behavior can be overridden by handling the preventable selectAllChange event.
selectAll
BooleanDefault: false