Version: 1.4 (the one that comes with PostgreSQL 8.1.2-1 by default)
OS: Windows XP Pro, SP2
In the Query editor Undo and Redo buttons change its state only in case if
editor window has been unfocused and then focused again.
For example:
1. Open Query editor
2. Write something
3. Press Ctrl-z
At this moment Redo button should be active, but it's not.
4. Switch to another window
5. Switch back
6. Redo is active
The same behavior is seen in the previous versions as well.