To manually test this, drag the text into the contenteditable, and then into the textarea. Observe the output. You should see a pair of insertFromDrop events dispatched on richDestination followed by another pair of insertFromDrop events dispatched on plainDestination.

WebKit
WebKit

(richDestination): type=beforeinput, inputType=insertFromDrop, data="null" dataTransfer={rich:WebKit plain:WebKit}
(richDestination): type=input, inputType=insertFromDrop, data="null" dataTransfer={rich:WebKit plain:WebKit}
(plainDestination): type=beforeinput, inputType=insertFromDrop, data="WebKit" dataTransfer=(null)
(plainDestination): type=input, inputType=insertFromDrop, data="WebKit" dataTransfer=(null)