After a certain height, the markdown editor should switch to scrolling. If the editor is too tall, then the toolbar is far away and it generally is just a lot bigger than normal expectations for text editing.
Method to do this: https://groups.google.com/d/msg/codemirror/1zjPxioENiA/ujiZU3Of-5oJ
With some preliminary tests, this makes [#7948] less of a problem too
db/7950
Looks fine in most places, but looks weird on the wiki edit page http://www.awesomescreenshot.com/image/468200/8dd4ef3eb7b23294190be5bf93bb7743
Good catch. Fixed on db/7950