Put views appended via appendToLinesView in the light DOM

This adds an insertion point to the lines div via a <content> tag,
allowing immediate children of the editor tag to be positioned relative
to the lines div but still be styled via global CSS.

src/lines-component.coffee

@@ -57,6 +57,10 @@ LinesComponent = React.createClass
     @lineIdsByScreenRow = {}
     @renderedDecorationsByLineId = {}
 
+  componentDidMount: ->
+    node = @getDOMNode()
+    node.appendChild(document.createElement('content'))
+
   shouldComponentUpdate: (newProps) ->
     return true unless isEqualForProperties(newProps, @props,
       'renderedRowRange', 'lineDecorations', 'highlightDecorations', 'lineHeightInPixels', 'defaultCharWidth',

src/text-editor-view.coffee

@@ -169,7 +169,7 @@ class TextEditorView extends View
   appendToLinesView: (view) ->
     view.css('position', 'absolute')
     view.css('z-index', 1)
-    @find('.lines').prepend(view)
+    @append(view)
 
   unmountComponent: ->
     React.unmountComponentAtNode(@element) if @component.isMounted()