Optimization: Use absolute positioning instead of flexbox in editor

Flexbox was causing layouts and repaints to cover the entire scroll view instead of just the edited line. This cuts down on DOM manipulation cost significantly.

src/app/editor.coffee

@@ -19,10 +19,9 @@ class Editor extends View
   @content: (params) ->
     @div class: @classes(params), tabindex: -1, =>
       @input class: 'hidden-input', outlet: 'hiddenInput'
-      @div class: 'flexbox', =>
-        @subview 'gutter', new Gutter
-        @div class: 'scroll-view', outlet: 'scrollView', =>
-          @div class: 'lines', outlet: 'renderedLines', =>
+      @subview 'gutter', new Gutter
+      @div class: 'scroll-view', outlet: 'scrollView', =>
+        @div class: 'lines', outlet: 'renderedLines', =>
       @div class: 'vertical-scrollbar', outlet: 'verticalScrollbar', =>
         @div outlet: 'verticalScrollbarContent'
 
@@ -135,7 +134,7 @@ class Editor extends View
       'page-down': @pageDown
       'page-up': @pageUp
 
-    if not @mini
+    unless @mini
       _.extend editorBindings,
         'save': @save
         'newline-below': @insertNewlineBelow
@@ -154,7 +153,6 @@ class Editor extends View
         'show-previous-buffer': @loadPreviousEditSession
         'toggle-line-comments': @toggleLineCommentsInSelection
 
-
     for name, method of editorBindings
       do (name, method) =>
         @on name, => method.call(this); false
@@ -331,6 +329,10 @@ class Editor extends View
     @verticalScrollbar.on 'scroll', =>
       @scrollTop(@verticalScrollbar.scrollTop(), adjustVerticalScrollbar: false)
 
+    unless @mini
+      @gutter.widthChanged = (newWidth) =>
+        @scrollView.css('left', newWidth + 'px')
+
     @scrollView.on 'scroll', =>
       if @scrollView.scrollLeft() == 0
         @gutter.removeClass('drop-shadow')
@@ -680,7 +682,7 @@ class Editor extends View
     @height(@lineHeight) if @mini
     fragment.remove()
 
-    @gutter.calculateDimensions()
+    @gutter.calculateWidth()
 
   prepareForScrolling: ->
     @adjustHeightOfRenderedLines()