200210230100, MDI, JDK1.4.1b21.
(This bug may be related to, or duplicate of issue
#25337.)
Steps to reproduce:
1. Open any java file.
2. Try to use go-to-source (Alt-o) feature. The
appropriate class should be opened, focused, the
caret should be moved at appropriate line.
3. After all of this is done, the focus is moved
to the tab of current editor pane.
4. When you press Left arrow or Right arrow, the
next/previous pane gets selected, and focus is
moved to appropriate editor.
5. Now, when you write, you write into the editor.

I have to say that this makes go-to-source feature
very hard to use. 
This used to work OK in 200210160100, so I think
this is a regression.