set active region before calculating bounds

It looks not like a setter for me, it's more an `updateActiveRegion` or `recalcActiveRegion`, because it gets no argument to set to. 

Also, for now, we try to get rid from long method names, so I suggest to name it just `region()` or smth. And the variable, if we get rid from underscored stuff and we're ok with new `$`-fashion, could be called `$region`, not to overlap with method name.

...Plus, we use mostly 4-space indent now.