Shift-meta-z to redo.

spec/atom/editor-spec.coffee

@@ -596,12 +596,15 @@ describe "Editor", ->
           editor.trigger keydownEvent('delete')
           expect(buffer.getLine(12)).toBe '};'
 
-    describe "when undo events are triggered on the editor", ->
-      it "undoes the last change", ->
+    describe "when undo/redo events are triggered on the editor", ->
+      it "undoes/redoes the last change", ->
         buffer.insert [0, 0], "foo"
         editor.trigger 'undo'
         expect(buffer.getLine(0)).not.toContain "foo"
 
+        editor.trigger 'redo'
+        expect(buffer.getLine(0)).toContain "foo"
+
     describe "when multiple lines are removed from the buffer (regression)", ->
       it "removes all of them from the dom", ->
         buffer.change(new Range([6, 24], [12, 0]), '')

spec/atom/undo-manager-spec.coffee

@@ -26,5 +26,36 @@ describe "UndoManager", ->
       undoManager.undo()
       expect(buffer.getLine(0)).toContain 'quicksort'
 
-    it "does not throw an exception when there is no last change", ->
+    it "does not throw an exception when there is nothing to undo", ->
       undoManager.undo()
+
+  describe ".redo()", ->
+    beforeEach ->
+      buffer.change(new Range([0, 5], [0, 9]), '')
+      buffer.insert([0, 6], 'h')
+      buffer.insert([0, 10], 'y')
+      undoManager.undo()
+      undoManager.undo()
+      expect(buffer.getLine(0)).toContain 'qsort'
+
+    it "redoes the last undone change", ->
+      undoManager.redo()
+      expect(buffer.getLine(0)).toContain 'qshort'
+
+      undoManager.redo()
+      expect(buffer.getLine(0)).toContain 'qshorty'
+
+      undoManager.undo()
+      expect(buffer.getLine(0)).toContain 'qshort'
+
+    it "does not throw an exception when there is nothing to redo", ->
+      undoManager.redo()
+      undoManager.redo()
+      undoManager.redo()
+
+    it "discards the redo history when there is a new change following an undo", ->
+      buffer.insert([0, 6], 'p')
+      expect(buffer.getText()).toContain 'qsport'
+
+      undoManager.redo()
+      expect(buffer.getText()).toContain 'qsport'

src/atom/editor.coffee

@@ -51,6 +51,7 @@ class Editor extends View
       'meta-c': 'copy'
       'meta-v': 'paste'
       'meta-z': 'undo'
+      'meta-Z': 'redo'
 
     @on 'move-right', => @moveCursorRight()
     @on 'move-left', => @moveCursorLeft()
@@ -67,6 +68,7 @@ class Editor extends View
     @on 'copy', => @copySelection()
     @on 'paste', => @paste()
     @on 'undo', => @undo()
+    @on 'redo', => @redo()
 
   buildCursorAndSelection: ->
     @cursor = new Cursor(this)
@@ -244,3 +246,6 @@ class Editor extends View
 
   undo: ->
     @undoManager.undo()
+
+  redo: ->
+    @undoManager.redo()

src/atom/undo-manager.coffee

@@ -1,17 +1,30 @@
 module.exports =
 class UndoManager
   undoHistory: null
-  undoInProgress: null
+  redoHistory: null
+  preserveHistory: false
 
   constructor: (@buffer) ->
     @undoHistory = []
+    @redoHistory = []
     @buffer.on 'change', (op) =>
-      @undoHistory.push(op) unless @undoInProgress
+      unless @preserveHistory
+        @undoHistory.push(op)
+        @redoHistory = []
 
   undo: ->
-    return unless @undoHistory.length
-    op = @undoHistory.pop()
-    @undoInProgress = true
-    @buffer.change op.newRange, op.oldText
-    @undoInProgress = false
+    if op = @undoHistory.pop()
+      @preservingHistory =>
+        @buffer.change op.newRange, op.oldText
+        @redoHistory.push op
 
+  redo: ->
+    if op = @redoHistory.pop()
+      @preservingHistory =>
+        @buffer.change op.oldRange, op.newText
+        @undoHistory.push op
+
+  preservingHistory: (fn) ->
+    @preserveHistory = true
+    fn()
+    @preserveHistory = false