Track AtomWindow from ipc open event

This ensures the window where the file is dropped is used to open
that file into even when that window does not have focus.

src/browser/atom-application.coffee

@@ -205,13 +205,15 @@ class AtomApplication
 
     # A request from the associated render process to open a new render process.
     ipc.on 'open', (event, options) =>
+      window = @windowForEvent(event)
       if options?
         if options.pathsToOpen?.length > 0
+          options.window = window
           @openPaths(options)
         else
           new AtomWindow(options)
       else
-        @promptForPath()
+        @promptForPath({window})
 
     ipc.on 'update-application-menu', (event, template, keystrokesByCommand) =>
       @applicationMenu.update(template, keystrokesByCommand)
@@ -290,6 +292,11 @@ class AtomApplication
     for atomWindow in @windows
       return atomWindow if atomWindow.containsPath(pathToOpen)
 
+  # Returns the {AtomWindow} for the given ipc event.
+  windowForEvent: ({sender}) ->
+    window = BrowserWindow.fromWebContents(sender)
+    _.find @windows, ({browserWindow}) -> window is browserWindow
+
   # Public: Returns the currently focused {AtomWindow} or undefined if none.
   focusedWindow: ->
     _.find @windows, (atomWindow) -> atomWindow.isFocused()
@@ -302,9 +309,10 @@ class AtomApplication
   #   :newWindow - Boolean of whether this should be opened in a new window.
   #   :devMode - Boolean to control the opened window's dev mode.
   #   :safeMode - Boolean to control the opened window's safe mode.
-  openPaths: ({pathsToOpen, pidToKillWhenClosed, newWindow, devMode, safeMode}) ->
+  #   :window - {AtomWindow} to open file paths in.
+  openPaths: ({pathsToOpen, pidToKillWhenClosed, newWindow, devMode, safeMode, window}) ->
     for pathToOpen in pathsToOpen ? []
-      @openPath({pathToOpen, pidToKillWhenClosed, newWindow, devMode, safeMode})
+      @openPath({pathToOpen, pidToKillWhenClosed, newWindow, devMode, safeMode, window})
 
   # Public: Opens a single path, in an existing window if possible.
   #
@@ -315,13 +323,14 @@ class AtomApplication
   #   :devMode - Boolean to control the opened window's dev mode.
   #   :safeMode - Boolean to control the opened window's safe mode.
   #   :windowDimensions - Object with height and width keys.
-  openPath: ({pathToOpen, pidToKillWhenClosed, newWindow, devMode, safeMode, windowDimensions}={}) ->
+  #   :window - {AtomWindow} to open file paths in.
+  openPath: ({pathToOpen, pidToKillWhenClosed, newWindow, devMode, safeMode, windowDimensions, window}={}) ->
     {pathToOpen, initialLine, initialColumn} = @locationForPathToOpen(pathToOpen)
 
     # Open files in the focused window
     unless pidToKillWhenClosed or newWindow
       if fs.statSyncNoException(pathToOpen).isFile?()
-        existingWindow = @topWindow
+        existingWindow = window ? @topWindow
 
       existingWindow ?= @windowForPath(pathToOpen) unless devMode
 
@@ -455,7 +464,8 @@ class AtomApplication
   #              should be in dev mode or not.
   #   :safeMode - A Boolean which controls whether any newly opened windows
   #               should be in safe mode or not.
-  promptForPath: ({type, devMode, safeMode}={}) ->
+  #   :window - An {AtomWindow} to use for opening a selected file path.
+  promptForPath: ({type, devMode, safeMode, window}={}) ->
     type ?= 'all'
     properties =
       switch type
@@ -464,4 +474,4 @@ class AtomApplication
         when 'all' then ['openFile', 'openDirectory']
         else throw new Error("#{type} is an invalid type for promptForPath")
     dialog.showOpenDialog title: 'Open', properties: properties.concat(['multiSelections', 'createDirectory']), (pathsToOpen) =>
-      @openPaths({pathsToOpen, devMode, safeMode})
+      @openPaths({pathsToOpen, devMode, safeMode, window})