Put your playlist in the Songs view. Select the heading of left-most column of numbers to sort on it. This is the Play Order. Drag new content from other areas of the library and drop them onto the playlist in the sidebar. Select the playlist and drag songs up or down the list, or remove, as required. In short you can still edit playlists as before, even if the tools are slightly different. You can also open a playlist in a separate window if that helps.
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