I am assuming this is a bug since Bear.app can perform this behavior.
- Open Panda app
- Open Apple Mail
- In a message header, mouse over until your cursor becomes a hand.
- Click and drag the message to Panda
- while dragging the cursor shows a + sign.
- when you release in Panda, you will get a link for the message.
- when dragging the cursor shows a + sign.
- when you release in Panda, nothing is added to the page.