Undoing in secondary area returns focus to primary area

[ageorgou]

To reproduce:
Split window horizontally or vertically (not with the Arabic pane!)
Make edits in the secondary area
Undo while still in the secondary area. The focus will move to the primary area.

This would be particularly annoying with a large file when the two areas show different parts. Undoing from the secondary area will then make the primary area scroll up/down to match the position in the secondary area.

This has to do with how we are identifying the source of an editing event, and the fact that the two areas share the same underlying document, so it's not easy to distinguish between them.