mercurial/vfs.py

--- a/mercurial/vfs.py	Thu Oct 12 19:20:04 2017 -0700
+++ b/mercurial/vfs.py	Thu Oct 12 23:30:46 2017 -0700
@@ -543,7 +543,7 @@
 
         # Only Windows/NTFS has slow file closing. So only enable by default
         # on that platform. But allow to be enabled elsewhere for testing.
-        defaultenabled = pycompat.osname == 'nt'
+        defaultenabled = pycompat.iswindows
         enabled = ui.configbool('worker', 'backgroundclose', defaultenabled)
 
         if not enabled: