I had 2 different versions of drag and drop in my code. The new one was HTML5 based, using the modern APIs. The old one was using an old-school "draggable" class, and assorted code.

Within a few minutes, I had ported over the newer style to the older code. As I tested, I found lots of small UI tweaks to make it smoother:

• Hide the other menus from opening as we drag/drop
• Correctly select the new row that we just dropped
• (Built-in) have a high-fidelity preview of where the new element will appear

Just getting something working would have taken 30 minutes before, and I wouldn't have wanted to start adding polish. Now, I can see the UI emerge before me, notice a little corner case, and tweak it quickly.