Opening a previously opened url restores the same buffer and session.

spec/atom/editor-spec.coffee

@@ -37,6 +37,18 @@ describe "Editor", ->
       fileContents = fs.read(filePath)
       expect(editor.getAceSession().getValue()).toBe fileContents
 
+    it "restores the ace edit session for a previously assigned buffer", ->
+      buffer = new Buffer filePath
+      editor.setBuffer buffer
+
+      aceSession = editor.getAceSession()
+
+      editor.setBuffer new Buffer(tempFilePath)
+      expect(editor.getAceSession()).not.toBe(aceSession)
+
+      editor.setBuffer(buffer)
+      expect(editor.getAceSession()).toBe aceSession
+
     it "sets the language mode based on the file extension", ->
       buffer = new Buffer "something.js"
       editor.setBuffer buffer

spec/atom/project-spec.coffee

@@ -19,6 +19,11 @@ describe "Project", ->
     beforeEach ->
       absolutePath = require.resolve('fixtures/dir/a')
 
+    it "always returns the same buffer for the same canonical path", ->
+      buffer = project.open(absolutePath)
+      expect(project.open(absolutePath)).toBe buffer
+      expect(project.open('a')).toBe buffer
+
     describe "when given an absolute path", ->
       it "returns a buffer for the given path", ->
         expect(project.open(absolutePath).url).toBe absolutePath
@@ -27,3 +32,10 @@ describe "Project", ->
       it "returns a buffer for the given path (relative to the project root)", ->
         expect(project.open('a').url).toBe absolutePath
 
+  describe ".resolve(path)", ->
+    it "returns an absolute path based on the project's root", ->
+      absolutePath = require.resolve('fixtures/dir/a')
+      expect(project.resolve('a')).toBe absolutePath
+      expect(project.resolve(absolutePath + '/../a')).toBe absolutePath
+      expect(project.resolve('a/../a')).toBe absolutePath
+

spec/stdlib/fs-spec.coffee

@@ -25,15 +25,13 @@ describe "fs", ->
 
       describe "when recursive is true", ->
         it "returns a promise that resolves to the recursive contents of that directory that are files", ->
-          waitsFor (complete) ->
+          waitsForPromise ->
             fs.async.listFiles(directoryPath, true).done (result) ->
               expect(result).toEqual (path for path in fs.list(directoryPath, true) when fs.isFile(path))
-              complete()
 
       describe "when recursive is false", ->
         it "returns a promise that resolves to the contents of that directory that are files", ->
-          waitsFor (complete) ->
+          waitsForPromise ->
             fs.async.listFiles(directoryPath).done (result) ->
               expect(result).toEqual (path for path in fs.list(directoryPath) when fs.isFile(path))
-              complete()
 

src/atom/editor.coffee

@@ -14,6 +14,7 @@ class Editor extends Template
     buffer: null
 
     initialize: () ->
+      @aceSessions = {}
       @buildAceEditor()
       @setBuffer(new Buffer)
 
@@ -24,8 +25,10 @@ class Editor extends Template
       @aceEditor.destroy()
 
     setBuffer: (@buffer) ->
-      session = new EditSession(@buffer.aceDocument, @buffer.getMode())
-      @aceEditor.setSession(session)
+      @aceEditor.setSession @getAceSessionForBuffer(buffer)
+
+    getAceSessionForBuffer: (buffer) ->
+      @aceSessions[@buffer.url] ?= new EditSession(@buffer.aceDocument, @buffer.getMode())
 
     buildAceEditor: ->
       @aceEditor = ace.edit this[0]

src/atom/project.coffee

@@ -4,17 +4,19 @@ Buffer = require 'buffer'
 module.exports =
 
 class Project
+  buffers: null
+
   constructor: (@url) ->
+    @buffers = {}
 
   getFilePaths: ->
     fs.async.listFiles(@url, true)
 
   open: (filePath) ->
-    new Buffer(@resolve(filePath))
+    filePath = @resolve filePath
+    @buffers[filePath] ?= new Buffer(filePath)
 
   resolve: (filePath) ->
-    if filePath[0] == '/'
-      filePath
-    else
-      fs.join(@url, filePath)
+    filePath = fs.join(@url, filePath) unless filePath[0] == '/'
+    fs.absolute filePath