Reassign scrollTop if deleting lines decreases the max scrollTop

Fixes #2725

spec/display-buffer-spec.coffee

@@ -47,6 +47,15 @@ describe "DisplayBuffer", ->
       buffer.insert([0,0], oneHundredLines)
       expect(displayBuffer.getLineCount()).toBe 100 + originalLineCount
 
+    it "reassigns the scrollTop if it exceeds the max possible value after lines are removed", ->
+      displayBuffer.manageScrollPosition = true
+      displayBuffer.setHeight(50)
+      displayBuffer.setLineHeightInPixels(10)
+      displayBuffer.setScrollTop(80)
+
+      buffer.delete([[8, 0], [10, 0]])
+      expect(displayBuffer.getScrollTop()).toBe 60
+
   describe "soft wrapping", ->
     beforeEach ->
       displayBuffer.setSoftWrap(true)

src/display-buffer.coffee

@@ -182,10 +182,13 @@ class DisplayBuffer extends Model
   getScrollTop: -> @scrollTop
   setScrollTop: (scrollTop) ->
     if @manageScrollPosition
-      @scrollTop = Math.round(Math.max(0, Math.min(@getScrollHeight() - @getClientHeight(), scrollTop)))
+      @scrollTop = Math.round(Math.max(0, Math.min(@getMaxScrollTop(), scrollTop)))
     else
       @scrollTop = Math.round(scrollTop)
 
+  getMaxScrollTop: ->
+    @getScrollHeight() - @getClientHeight()
+
   getScrollBottom: -> @scrollTop + @height
   setScrollBottom: (scrollBottom) ->
     @setScrollTop(scrollBottom - @getClientHeight())
@@ -981,6 +984,7 @@ class DisplayBuffer extends Model
   handleTokenizedBufferChange: (tokenizedBufferChange) =>
     {start, end, delta, bufferChange} = tokenizedBufferChange
     @updateScreenLines(start, end + 1, delta, delayChangeEvent: bufferChange?)
+    @setScrollTop(Math.min(@getScrollTop(), @getMaxScrollTop())) if delta < 0
 
   updateScreenLines: (startBufferRow, endBufferRow, bufferDelta=0, options={}) ->
     startBufferRow = @rowMap.bufferRowRangeForBufferRow(startBufferRow)[0]