"Undo" is disabled after drag&drop of a whole sequence

Scenario:

1. Open "samples/CLUSTALW/COI.aln".
2. Select a region in a sequence and copy it (Ctrl+C).
3. Paste it (Ctrl+V).
   Expected result: a new sequence appears at the end of the sequence list (it has name "clipboard...").
4. Select the whole added sequence (using a mouse or the Shift key).
5. Drag and drop this sequence a bit.
   Expected result: the "Undo" button is enabled.
   Actual result:  the "Undo" button is disabled.

Workaround: the "Undo" button can become enabled, for example, if a gap is inserted into this or other sequence. After that "Undo" works as expected: the first click removes the gap, the second one restores the clipboard sequence position.