SelectOpaque is added to the current selection instead of replacing it. It is not convenient in my opinion.
New artists not understand why not works SelectOpaque if SelectAll used before activating this feature. Because artists must to deselect selection completely and only then choose SelectOpaque.
Note: for experienced artists needs a fast access for SelectOpaque. Like: Ctrl + ClickOnLayerPicture (in LayersDocker). [for discussion]