I figured out that the pleco app supports x-callback-url functionality.
Is there any way to open the pasteboard reader instead of the search field for sending whole sentences/paragraphs to the pleco app?
If you leave the query section blank and set Pleco to pop up the pasteboard reader automatically (Settings / General) then it should come up, but there won't be a "Return To" button since we only have UI for that in the dictionary screen.
