Implement shouldComponentUpdate for LinesComponent

We accumulate pending changes and pass them to the lines and the gutter
to help them determine whether to update. The lines only update if the
visible row range changed or if there was a change in the visible row
range.

src/lines-component.coffee

@@ -1,6 +1,6 @@
 React = require 'react'
 {div, span} = require 'reactionary'
-{debounce, isEqualForProperties, multiplyString} = require 'underscore-plus'
+{debounce, isEqual, isEqualForProperties, multiplyString} = require 'underscore-plus'
 {$$} = require 'space-pen'
 
 DummyLineNode = $$(-> @div className: 'line', style: 'position: absolute; visibility: hidden;', => @span 'x')[0]
@@ -28,6 +28,15 @@ LinesComponent = React.createClass
     @measuredLines = new WeakSet
     @updateModelDimensions()
 
+  shouldComponentUpdate: (newProps) ->
+    return true unless isEqualForProperties(newProps, @props,  'visibleRowRange', 'fontSize', 'fontFamily', 'lineHeight', 'showIndentGuide')
+
+    {visibleRowRange, pendingChanges} = newProps
+    for change in pendingChanges
+      return true unless change.end <= visibleRowRange.start or visibleRowRange.end <= change.start
+
+    false
+
   componentDidUpdate: (prevProps) ->
     @updateModelDimensions() unless isEqualForProperties(prevProps, @props, 'fontSize', 'fontFamily', 'lineHeight')
     @clearScopedCharWidths() unless isEqualForProperties(prevProps, @props, 'fontSize', 'fontFamily')