Adjust DisplayBuffer::longestScreenRow when lines are inserted/removed

Fixes #2810

The line corresponding to the longest screen row moves when lines are
inserted or removed above it, so we need to adjust it accordingly or we
won't always realize when a change affects the longest line.

spec/display-buffer-spec.coffee

@@ -643,6 +643,19 @@ describe "DisplayBuffer", ->
       buffer.delete([[6, 0], [6, 65]])
       expect(displayBuffer.getMaxLineLength()).toBe 62
 
+    it "correctly updates the location of the longest screen line when changes occur", ->
+      expect(displayBuffer.getLongestScreenRow()).toBe 6
+      buffer.delete([[0, 0], [2, 0]])
+      expect(displayBuffer.getLongestScreenRow()).toBe 4
+      buffer.delete([[4, 0], [5, 0]])
+
+      expect(displayBuffer.getLongestScreenRow()).toBe 1
+      expect(displayBuffer.getMaxLineLength()).toBe 62
+
+      buffer.delete([[2, 0], [4, 0]])
+      expect(displayBuffer.getLongestScreenRow()).toBe 1
+      expect(displayBuffer.getMaxLineLength()).toBe 62
+
   describe "::destroy()", ->
     it "unsubscribes all display buffer markers from their underlying buffer marker (regression)", ->
       marker = displayBuffer.markBufferPosition([12, 2])