When editor contains nested editable elements, in some situations, (#693)
When editor contains nested editable elements, in some situations, (#693)

the focus goes to those elements. In firefox, this causes issues
with keyboard navigation and therefore it must be prevented.
isubastiCadmus/slate,ashutoshrishi/slate,AlbertHilb/slate,ianstormtaylor/slate,6174/slate,ianstormtaylor/slate