Don't measure new lines when scrolling with the mousewheel

It impacts scrolling performance. We can measure when scrolling comes
to a halt.

src/editor-component.coffee

@@ -257,7 +257,7 @@ EditorComponent = React.createClass
     # after mousewheel events stop being fired. Removing the target before then
     # will cause scrolling to stop suddenly.
     @preservedScreenRow = @screenRowForNode(event.target)
-    @clearPreservedScreenRowAfterDelay ?= debounce(@clearPreservedScreenRow, 100)
+    @clearPreservedScreenRowAfterDelay ?= debounce(@clearPreservedScreenRow, 300)
     @clearPreservedScreenRowAfterDelay()
 
     # Only scroll in one direction at a time

src/lines-component.coffee

@@ -44,7 +44,7 @@ LinesComponent = React.createClass
   componentDidUpdate: (prevProps) ->
     @updateModelDimensions() unless isEqualForProperties(prevProps, @props, 'fontSize', 'fontFamily', 'lineHeight')
     @clearScopedCharWidths() unless isEqualForProperties(prevProps, @props, 'fontSize', 'fontFamily')
-    @measureCharactersInNewLines()
+    @measureCharactersInNewLines() unless @props.preservedScreenRow?
 
   updateModelDimensions: ->
     {editor} = @props