[TxMt] Bug: auto-unfolding when extending selection by line does not update scrollbar

[Gerd Knops]

To reproduce:

- Have some folded blocks
- Tripple-click-drag downwards to extend selection by lines
- Blocks are unfolded, but the scroll bar is not updated

The only way to get to the text that fell through the bottom is to  
manually fold/unfold something.

I'd take an alpha of avian over a bugfix though... Soon?

Gerd