Every Buffer gets its own UndoManager.

spec/atom/editor-spec.coffee

@@ -935,6 +935,26 @@ describe "Editor", ->
       expect(editor.scrollTop()).toBe previousScrollTop
       expect(editor.horizontalScroller.scrollLeft()).toBe previousScrollLeft
 
+    it "recalls the undo history of the buffer when it is re-assigned", ->
+      editor.insertText('xyz')
+
+      otherBuffer = new Buffer
+      editor.setBuffer(otherBuffer)
+      editor.insertText('abc')
+      expect(otherBuffer.lineForRow(0)).toBe 'abc'
+      editor.undo()
+      expect(otherBuffer.lineForRow(0)).toBe ''
+
+      editor.setBuffer(buffer)
+      editor.undo()
+      expect(buffer.lineForRow(0)).toBe 'var quicksort = function () {'
+      editor.redo()
+      expect(buffer.lineForRow(0)).toBe 'xyzvar quicksort = function () {'
+
+      editor.setBuffer(otherBuffer)
+      editor.redo()
+      expect(otherBuffer.lineForRow(0)).toBe 'abc'
+
   describe ".clipScreenPosition(point)", ->
     it "selects the nearest valid position to the given point", ->
       expect(editor.clipScreenPosition(row: 1000, column: 0)).toEqual(row: buffer.lastRow(), column: buffer.lineForRow(buffer.lastRow()).length)

src/atom/buffer.coffee

@@ -2,6 +2,7 @@ _ = require 'underscore'
 fs = require 'fs'
 Range = require 'range'
 EventEmitter = require 'event-emitter'
+UndoManager = require 'undo-manager'
 
 module.exports =
 class Buffer
@@ -16,6 +17,7 @@ class Buffer
       @setText(fs.read(@path))
     else
       @setText('')
+    @undoManager = new UndoManager(this)
 
   getText: ->
     @lines.join('\n')
@@ -94,6 +96,12 @@ class Buffer
     @lines[oldRange.start.row..oldRange.end.row] = newTextLines
     @trigger 'change', { oldRange, newRange, oldText, newText }
 
+  undo: ->
+    @undoManager.undo()
+
+  redo: ->
+    @undoManager.redo()
+
   save: ->
     if not @path then throw new Error("Tried to save buffer with no url")
     fs.write @path, @getText()

src/atom/editor.coffee

@@ -7,7 +7,6 @@ Renderer = require 'renderer'
 Point = require 'point'
 Range = require 'range'
 Selection = require 'selection'
-UndoManager = require 'undo-manager'
 EditSession = require 'edit-session'
 
 $ = require 'jquery'
@@ -34,7 +33,6 @@ class Editor extends View
   buffer: null
   highlighter: null
   renderer: null
-  undoManager: null
   autoIndent: null
   lineCache: null
 
@@ -165,7 +163,6 @@ class Editor extends View
     @saveEditSession() if @editSession
     document.title = @buffer.path
     @renderer = new Renderer(@buffer)
-    @undoManager = new UndoManager(@buffer)
     @renderLines()
     @gutter.renderLineNumbers()
 
@@ -379,10 +376,10 @@ class Editor extends View
     @selection.delete()
 
   undo: ->
-    @undoManager.undo()
+    @buffer.undo()
 
   redo: ->
-    @undoManager.redo()
+    @buffer.redo()
 
   destroyFold: (foldId) ->
     fold = @renderer.foldsById[foldId]