Return early in FakeLinesYardstick for unrendered rows

This allows us to have a more consistent behavior with the real LinesYardstick.

spec/fake-lines-yardstick.coffee

@@ -7,6 +7,7 @@ class FakeLinesYardstick
 
   prepareScreenRowsForMeasurement: ->
     @presenter.getPreMeasurementState()
+    @screenRows = new Set(@presenter.getScreenRows())
 
   getScopedCharacterWidth: (scopeNames, char) ->
     @getScopedCharacterWidths(scopeNames)[char]
@@ -34,6 +35,8 @@ class FakeLinesYardstick
     left = 0
     column = 0
 
+    return {top, left: 0} unless @screenRows.has(screenPosition.row)
+
     iterator = @model.tokenizedLineForScreenRow(targetRow).getTokenIterator()
     while iterator.next()
       characterWidths = @getScopedCharacterWidths(iterator.getScopes())