the hardest part about copying selected text from a webview is the act
of selecting.
that said, i have no problem copying to/from the clipboard. please refer to
the attached images.
technically, evaluatejavascript() in the context of a valuecallback does work,
but it isn't necessary, as the prompt to copy the selected text to the
clipboard is already sitting there for you. either way, you have to have
some kind of button to capture the selection in a timely manner.