Decide to measure gutter's width in gutter

The gutter is in a better position to determine if the max line number
length has changed because it's a property that gets passed in so we
can compare current with previous.

Fixes #2659

src/gutter-component.coffee

@@ -12,6 +12,7 @@ GutterComponent = React.createClass
   mixins: [SubscriberMixin]
 
   dummyLineNumberNode: null
+  measuredWidth: null
 
   render: ->
     {scrollHeight, scrollViewHeight, scrollTop} = @props
@@ -48,10 +49,13 @@ GutterComponent = React.createClass
     false
 
   componentDidUpdate: (oldProps) ->
-    unless oldProps.maxLineNumberDigits is @props.maxLineNumberDigits
+    unless isEqualForProperties(oldProps, @props, 'maxLineNumberDigits')
       @updateDummyLineNumber()
       @removeLineNumberNodes()
 
+    unless isEqualForProperties(oldProps, @props, 'maxLineNumberDigits', 'defaultCharWidth')
+      @measureWidth()
+
     @clearScreenRowCaches() unless oldProps.lineHeightInPixels is @props.lineHeightInPixels
     @updateLineNumbers()
 
@@ -202,6 +206,12 @@ GutterComponent = React.createClass
       else
         editor.foldBufferRow(bufferRow)
 
+  measureWidth: ->
+    width = @getDOMNode().offsetWidth
+    unless width is @measuredWidth
+      @measuredWidth = width
+      @props.onWidthChanged?(width)
+
 # Created because underscore uses === not _.isEqual, which we need
 contains = (array, target) ->
   return false unless array?