This is a pity. The Listbox with checkboxes also performs inconsistently with the standard browser context menu. Sometimes the checkmark is not changed, sometimes it is changed when trying to open the browser context menu. It seems to depend on how fast the mouse is released, if it is release rapidly the check status changes.
Anyhow, I guess it would be nice to be able to deactivate right-click checking/unchecking, maybe it even should be default. A user expects a context menu to not modifiy any data if no action from the menu is taken.
Kind regards, Claus