How to pin and keep window always open
By default, the clipboard history window closes when you switch to another app or paste an item. You can pin the window to keep it open on top of other windows.
To pin the window, click the pin button in the toolbar. Its tooltip says Always display.
While the window is pinned, it stays open when you switch to another app or paste an item. You can still close it by pressing Esc, ⌘W or ⇧⌘V.
To unpin the window, click the pin button again. Its tooltip says Stop always displaying.