Windows can be docked to side of screen for better usability or for better screen area utilisation. Several windows can be docked, with accordion-like switching. The window docker can be resized to fit user needs.
This feature is in development, please do not use it yet
echo $modal = Modal::widget([ 'id' => 'modal-1', 'title' => 'My modal', 'dockable' => true, 'content' => 'Some content' ]);