Multiple newlines can be inserted on the same line

Redesign Cursor.handleBufferChange. The basic intuition is that each cursor should have the same spatial relationship with the end of the new range as it previously had with the end of the old range.

spec/atom/editor-spec.coffee

@@ -770,36 +770,54 @@ describe "Editor", ->
       expect(cursor2.getBufferPosition()).toEqual [6, 0]
 
     describe "inserting text", ->
-      describe "when inserting characters other than newlines", ->
-        it "inserts text for all cursors", ->
-          editor.setCursorScreenPosition([3, 0])
-          editor.addCursorAtScreenPosition([6, 0])
+      describe "when cursors are on the same line", ->
+        describe "when inserting newlines", ->
+          it "breaks the line into three lines at the cursor locations", ->
+            editor.setCursorScreenPosition([3, 13])
+            editor.addCursorAtScreenPosition([3, 38])
 
-          editor.insertText("abc")
-          expect(editor.lineForBufferRow(3)).toBe "abc    var pivot = items.shift(), current, left = [], right = [];"
-          expect(editor.lineForBufferRow(6)).toBe "abc      current < pivot ? left.push(current) : right.push(current);"
+            editor.insertText('\n')
 
-          [cursor1, cursor2] = editor.compositeCursor.getCursors()
-          expect(cursor1.getBufferPosition()).toEqual [3,3]
-          expect(cursor2.getBufferPosition()).toEqual [6,3]
+            expect(editor.lineForBufferRow(3)).toBe "    var pivot"
+            expect(editor.lineForBufferRow(4)).toBe " = items.shift(), current"
+            expect(editor.lineForBufferRow(5)).toBe ", left = [], right = [];"
+            expect(editor.lineForBufferRow(6)).toBe "    while(items.length > 0) {"
 
-      describe "when inserting newlines", ->
-        it "inserts newlines for all cursors", ->
-          editor.setCursorScreenPosition([3, 0])
-          editor.addCursorAtScreenPosition([6, 0])
+            [cursor1, cursor2] = editor.compositeCursor.getCursors()
+            expect(cursor1.getBufferPosition()).toEqual [4, 0]
+            expect(cursor2.getBufferPosition()).toEqual [5, 0]
 
-          editor.insertText("\n")
-          expect(editor.lineForBufferRow(3)).toBe ""
-          expect(editor.lineForBufferRow(4)).toBe "    var pivot = items.shift(), current, left = [], right = [];"
-          expect(editor.lineForBufferRow(5)).toBe "    while(items.length > 0) {"
-          expect(editor.lineForBufferRow(6)).toBe "      current = items.shift();"
-          expect(editor.lineForBufferRow(7)).toBe ""
-          expect(editor.lineForBufferRow(8)).toBe "      current < pivot ? left.push(current) : right.push(current);"
-          expect(editor.lineForBufferRow(9)).toBe "    }"
+      describe "when cursors are on different lines", ->
+        describe "when inserting characters other than newlines", ->
+          it "inserts text for all cursors", ->
+            editor.setCursorScreenPosition([3, 0])
+            editor.addCursorAtScreenPosition([6, 0])
 
-          [cursor1, cursor2] = editor.compositeCursor.getCursors()
-          expect(cursor1.getBufferPosition()).toEqual [4,0]
-          expect(cursor2.getBufferPosition()).toEqual [8,0]
+            editor.insertText("abc")
+            expect(editor.lineForBufferRow(3)).toBe "abc    var pivot = items.shift(), current, left = [], right = [];"
+            expect(editor.lineForBufferRow(6)).toBe "abc      current < pivot ? left.push(current) : right.push(current);"
+
+            [cursor1, cursor2] = editor.compositeCursor.getCursors()
+            expect(cursor1.getBufferPosition()).toEqual [3,3]
+            expect(cursor2.getBufferPosition()).toEqual [6,3]
+
+        describe "when inserting newlines", ->
+          it "inserts newlines for all cursors", ->
+            editor.setCursorScreenPosition([3, 0])
+            editor.addCursorAtScreenPosition([6, 0])
+
+            editor.insertText("\n")
+            expect(editor.lineForBufferRow(3)).toBe ""
+            expect(editor.lineForBufferRow(4)).toBe "    var pivot = items.shift(), current, left = [], right = [];"
+            expect(editor.lineForBufferRow(5)).toBe "    while(items.length > 0) {"
+            expect(editor.lineForBufferRow(6)).toBe "      current = items.shift();"
+            expect(editor.lineForBufferRow(7)).toBe ""
+            expect(editor.lineForBufferRow(8)).toBe "      current < pivot ? left.push(current) : right.push(current);"
+            expect(editor.lineForBufferRow(9)).toBe "    }"
+
+            [cursor1, cursor2] = editor.compositeCursor.getCursors()
+            expect(cursor1.getBufferPosition()).toEqual [4,0]
+            expect(cursor2.getBufferPosition()).toEqual [8,0]
 
     describe "backspace", ->
       describe "when cursors are on the same line", ->

src/atom/cursor.coffee

@@ -17,11 +17,18 @@ class Cursor extends View
 
   handleBufferChange: (e) ->
     { newRange, oldRange } = e
-    delta = newRange.end.subtract(oldRange.end)
+    position = @getBufferPosition()
+    return if position.isLessThan(oldRange.end)
 
-    delta.column = 0 if newRange.end.row != @getBufferPosition().row
-    return if @getBufferPosition().isLessThan(oldRange.end)
-    @setBufferPosition(@getBufferPosition().add(delta))
+    newRow = newRange.end.row
+    newColumn = newRange.end.column
+    if position.row == oldRange.end.row
+      newColumn += position.column - oldRange.end.column
+    else
+      newColumn = position.column
+      newRow += position.row - oldRange.end.row
+
+    @setBufferPosition([newRow, newColumn])
 
   setScreenPosition: (position, options={}) ->
     position = Point.fromObject(position)