In the top part of most pages you can switch what is currently the Focused (global) Selection. If you try changing to another page after changing it, you'll notice that the selection will follow you to another page. You can always switch to <No Selection> to start over (but yes, previous selections will still be available in the combo box).

I take it that you're looking for a way to clear out all previous selections, and not just selecting the "<No Selection>"? If so, how would you want to access that functionality? A context menu with "Clear All Selections"? Something else? Please let me know what you'd prefer, and I'll open an issue and add it. :)