"Scroll" gutter by relatively positioning its line numbers

This matches how we scroll lines, and eliminates opportunities for the gutter to get out of sync with the lines. If that happened, it would probably be a bug, but this at least eliminates one source of confusion when debugging.

spec/app/editor-spec.coffee

@@ -337,13 +337,13 @@ describe "Editor", ->
         expect(editor.verticalScrollbar.scrollTop()).toBe 100
         expect(editor.scrollView.scrollTop()).toBe 0
         expect(editor.renderedLines.css('top')).toBe "-100px"
-        expect(editor.gutter.scrollTop()).toBe 100
+        expect(editor.gutter.lineNumbers.css('top')).toBe "-100px"
 
         editor.scrollTop(120)
         expect(editor.verticalScrollbar.scrollTop()).toBe 120
         expect(editor.scrollView.scrollTop()).toBe 0
         expect(editor.renderedLines.css('top')).toBe "-120px"
-        expect(editor.gutter.scrollTop()).toBe 120
+        expect(editor.gutter.lineNumbers.css('top')).toBe "-120px"
 
       it "does not allow negative scrollTops to be assigned", ->
         editor.scrollTop(-100)
@@ -365,7 +365,7 @@ describe "Editor", ->
           editor.scrollTop(100, adjustVerticalScrollbar: false)
           expect(editor.verticalScrollbar.scrollTop()).toBe 0
           expect(editor.renderedLines.css('top')).toBe "-100px"
-          expect(editor.gutter.scrollTop()).toBe 100
+          expect(editor.gutter.lineNumbers.css('top')).toBe "-100px"
 
     describe "when called with no argument", ->
       it "returns the last assigned value or 0 if none has been assigned", ->

src/app/editor.coffee

@@ -413,7 +413,7 @@ class Editor extends View
     @updateRenderedLines() if @attached
 
     @renderedLines.css('top', -scrollTop)
-    @gutter.scrollTop(scrollTop)
+    @gutter.lineNumbers.css('top', -scrollTop)
     if options?.adjustVerticalScrollbar ? true
       @verticalScrollbar.scrollTop(scrollTop)
 

src/app/gutter.coffee

@@ -7,7 +7,7 @@ module.exports =
 class Gutter extends View
   @content: ->
     @div class: 'gutter', =>
-      @div outlet: 'lineNumbers', class: 'wtf'
+      @div outlet: 'lineNumbers', class: 'line-numbers'
 
   editor: ->
     editor = @parentView

static/editor.css

@@ -27,6 +27,10 @@
   text-align: right;
 }
 
+.editor .gutter .line-numbers {
+  position: relative;
+}
+
 .editor.mini .gutter {
   display: none;
 }