When testing an application that used a JEditorPane with a customized EditorKit for content of the type text/java, I found that cut and copy operations were failing to put anything on the system clipboard.

It turns out that BasicTextUI.TextTransferable.isPlainSupported() is only returning true when the content is text/plain. Since BasicTextUI is used for text components, it seams reasonable that we should always support text/plain, and thus always return true from this method.