@BoucherS: Please make sure that the new "select All" option will:
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call the Command Property's Execute Method and Raise the ItemSelectionChanged Event for every item. Concerned since CheckComboBox is not doing so if SelectedItemsOverride is modified prior to the drop-down opening (as reported in this Issue: CheckComboBox : Command not raised if popup not opened).
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have a programmatic way to be checked / unchecked.
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work like it does in Excel Column Filters (e.g. a) if checked / unchecked, automatically checks / unchecks all items, b) allows override via checking / unchecking individual items afterwards and b) gets automatically checked / unchecked if all / < all items are explicitly selected).