Preserve original selection's range when adding selection's below

Just like the cursor tries to stay in its "goal column" when moving
vertically, here we try to keep the same selection even when adding
across shorter lines.

spec/app/edit-session-spec.coffee

@@ -720,6 +720,18 @@ describe "EditSession", ->
           [[4, 25], [4, 29]]
         ]
 
+      it "honors the original selection's region when adding across shorter lines", ->
+        editSession.setSelectedBufferRange([[3, 22], [3, 38]])
+        editSession.addSelectionBelow()
+        editSession.addSelectionBelow()
+        editSession.addSelectionBelow()
+        expect(editSession.getSelectedBufferRanges()).toEqual [
+          [[3, 22], [3, 38]]
+          [[4, 22], [4, 29]]
+          [[5, 22], [5, 30]]
+          [[6, 22], [6, 38]]
+        ]
+
     describe "when the cursor is moved while there is a selection", ->
       makeSelection = -> selection.setBufferRange [[1, 2], [1, 5]]
 

src/app/edit-session.coffee

@@ -585,7 +585,7 @@ class EditSession
     unless options.preserveFolds
       @destroyFoldsIntersectingBufferRange(@getMarkerBufferRange(marker))
     cursor = @addCursor(marker)
-    selection = new Selection({editSession: this, marker, cursor})
+    selection = new Selection(_.extend({editSession: this, marker, cursor}, options))
     @selections.push(selection)
     selectionBufferRange = selection.getBufferRange()
     @mergeIntersectingSelections()
@@ -600,7 +600,7 @@ class EditSession
   addSelectionForBufferRange: (bufferRange, options={}) ->
     options = _.defaults({invalidationStrategy: 'never'}, options)
     marker = @markBufferRange(bufferRange, options)
-    @addSelection(marker)
+    @addSelection(marker, options)
 
   setSelectedBufferRange: (bufferRange, options) ->
     @setSelectedBufferRanges([bufferRange], options)

src/app/selection.coffee

@@ -5,10 +5,12 @@ _ = require 'underscore'
 module.exports =
 class Selection
   wordwise: false
+  editSession: null
   initialScreenRange: null
+  goalBufferRange: null
   needsAutoscroll: null
 
-  constructor: ({@cursor, @marker, @editSession}) ->
+  constructor: ({@cursor, @marker, @editSession, @goalBufferRange}) ->
     @cursor.selection = this
     @editSession.observeMarker @marker, => @screenRangeChanged()
     @cursor.on 'destroyed.selection', =>
@@ -149,10 +151,10 @@ class Selection
     @modifySelection => @cursor.moveToEndOfWord()
 
   addSelectionBelow: ->
-    range = @getBufferRange().copy()
+    range = (@goalBufferRange ? @getBufferRange()).copy()
     range.start.row++
     range.end.row++
-    @editSession.addSelectionForBufferRange(range)
+    @editSession.addSelectionForBufferRange(range, goalBufferRange: range)
 
   insertText: (text, options={}) ->
     oldBufferRange = @getBufferRange()