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])

src/display-buffer.coffee

@@ -593,6 +593,12 @@ class DisplayBuffer extends Model
   getMaxLineLength: ->
     @maxLineLength
 
+  # Gets the row number of the longest screen line.
+  #
+  # Return a {}
+  getLongestScreenRow: ->
+    @longestScreenRow
+
   # Given a buffer position, this converts it into a screen position.
   #
   # bufferPosition - An object that represents a buffer position. It can be either
@@ -991,18 +997,19 @@ class DisplayBuffer extends Model
     endBufferRow = @rowMap.bufferRowRangeForBufferRow(endBufferRow - 1)[1]
     startScreenRow = @rowMap.screenRowRangeForBufferRow(startBufferRow)[0]
     endScreenRow = @rowMap.screenRowRangeForBufferRow(endBufferRow - 1)[1]
-
     {screenLines, regions} = @buildScreenLines(startBufferRow, endBufferRow + bufferDelta)
+    screenDelta = screenLines.length - (endScreenRow - startScreenRow)
+
     @screenLines[startScreenRow...endScreenRow] = screenLines
     @rowMap.spliceRegions(startBufferRow, endBufferRow - startBufferRow, regions)
-    @findMaxLineLength(startScreenRow, endScreenRow, screenLines)
+    @findMaxLineLength(startScreenRow, endScreenRow, screenLines, screenDelta)
 
     return if options.suppressChangeEvent
 
     changeEvent =
       start: startScreenRow
       end: endScreenRow - 1
-      screenDelta: screenLines.length - (endScreenRow - startScreenRow)
+      screenDelta: screenDelta
       bufferDelta: bufferDelta
 
     if options.delayChangeEvent
@@ -1057,7 +1064,7 @@ class DisplayBuffer extends Model
 
     {screenLines, regions}
 
-  findMaxLineLength: (startScreenRow, endScreenRow, newScreenLines) ->
+  findMaxLineLength: (startScreenRow, endScreenRow, newScreenLines, screenDelta) ->
     oldMaxLineLength = @maxLineLength
 
     if startScreenRow <= @longestScreenRow < endScreenRow
@@ -1066,13 +1073,15 @@ class DisplayBuffer extends Model
       maxLengthCandidatesStartRow = 0
       maxLengthCandidates = @screenLines
     else
+      @longestScreenRow += screenDelta if endScreenRow < @longestScreenRow
       maxLengthCandidatesStartRow = startScreenRow
       maxLengthCandidates = newScreenLines
 
-    for screenLine, screenRow in maxLengthCandidates
+    for screenLine, i in maxLengthCandidates
+      screenRow = maxLengthCandidatesStartRow + i
       length = screenLine.text.length
       if length > @maxLineLength
-        @longestScreenRow = maxLengthCandidatesStartRow + screenRow
+        @longestScreenRow = screenRow
         @maxLineLength = length
 
     @computeScrollWidth() if oldMaxLineLength isnt @maxLineLength