After pasting text that gets (or could get) transformed into the new WikiText editor, the text input cursor is repositioned to the start of the textarea, and stops blinking (ie. it seems to disappear - while the textarea doesn't actually get unfocused, it looks just as if it happened). Pressing undo will not cause the old position to be recovered, either.

To reproduce, copy some characters from a section heading of a Wikipedia article and paste them into the editor.