Minor fixes for canvas and keyCapture to change tree direction

Minor stuff to deal with scrolling issues in the exercise demo.

Also adds a keyCapture for changing the tree direction. Not perfect (when a review tree is shown, both trees are triggered once we hit the specified key), but OK for now.

`state` did reinits of keyCaptures which is not a good idea: We would have them defined twice then and also trigger them twice once we hit a key.