Reuse current window if it has no project path

Fixes #5615

spec/integration/startup-spec.coffee

@@ -121,3 +121,13 @@ describe "Starting Atom", ->
                 projectPaths.push(value[0])
               .then ->
                 expect(projectPaths.sort()).toEqual([tempDirPath, otherTempDirPath].sort())
+
+    it "opens the path in the current window if it doesn't have a project path yet", ->
+      runAtom [], {ATOM_HOME: AtomHome}, (client) ->
+        client
+          .waitForExist("atom-workspace")
+          .startAnotherAtom([tempDirPath], ATOM_HOME: AtomHome)
+          .waitUntil((->
+            @title()
+              .then(({value}) -> value.indexOf(path.basename(tempDirPath)) >= 0)), 5000)
+          .waitForWindowCount(1, 5000)

src/browser/atom-application.coffee

@@ -360,8 +360,13 @@ class AtomApplication
       existingWindow = @windowForPaths(pathsToOpen, devMode)
 
       # Default to using the specified window or the last focused window
+      currentWindow = window ? @lastFocusedWindow
+
       if pathsToOpen.every((pathToOpen) -> fs.statSyncNoException(pathToOpen).isFile?())
-        existingWindow ?= window ? @lastFocusedWindow
+        existingWindow ?= currentWindow
+
+      unless currentWindow?.projectPaths?.length > 0
+        existingWindow ?= currentWindow
 
     if existingWindow?
       openedWindow = existingWindow