Step 1. Open a long topic with > 100 messages:
Step 2. Select ALL messages (use wrench). It will say, e.g. 100 messages selected:
Step 3. De-select the first message. Now it says: 19 messages selected:
I suppose that’s because only 20 messages are loaded initially on the page, and when you deselect one message, there are 19 messages remaining.
Now, if you proceed further and move the messages, only 19 messages will be moved.