When you drag a text line that is wider than the panel window, the panel scrolls to the extreme right position, which makes it impossible to see the drop target nodes (which have now scrolled off the left of the screen).
Adding horizontal scrolling, like the vertical scrolling, while dragging would be one solution. A simpler one (for the user) would be to suppress the scroll right as you pick up a long line.
Workaround: ensure shorter lines or a wider panel.