diff --git a/python/ycm/client/command_request.py b/python/ycm/client/command_request.py
index 6311d308..55be6e9d 100644
--- a/python/ycm/client/command_request.py
+++ b/python/ycm/client/command_request.py
@@ -69,6 +69,8 @@ class CommandRequest( BaseRequest ):
       self._HandleFixitResponse()
     elif 'message' in self._response:
       self._HandleMessageResponse()
+    elif 'detailed_info' in self._response:
+      self._HandleDetailedInfoResponse()
 
 
   def _HandleGotoResponse( self ):
@@ -99,6 +101,10 @@ class CommandRequest( BaseRequest ):
     vimsupport.EchoText( self._response[ 'message' ] )
 
 
+  def _HandleDetailedInfoResponse( self ):
+    vimsupport.WriteToPreviewWindow( self._response[ 'detailed_info' ] )
+
+
 def SendCommandRequest( arguments, completer ):
   request = CommandRequest( arguments, completer )
   # This is a blocking call.
diff --git a/python/ycm/client/completion_request.py b/python/ycm/client/completion_request.py
index 4cb2abd9..584a9bd0 100644
--- a/python/ycm/client/completion_request.py
+++ b/python/ycm/client/completion_request.py
@@ -21,10 +21,10 @@ from ycmd.utils import ToUtf8IfNeeded
 from ycm.client.base_request import ( BaseRequest, JsonFromFuture,
                                       HandleServerException,
                                       MakeServerException )
-import os
 
 TIMEOUT_SECONDS = 0.5
 
+
 class CompletionRequest( BaseRequest ):
   def __init__( self, request_data ):
     super( CompletionRequest, self ).__init__()
@@ -85,7 +85,7 @@ def ConvertCompletionDataToVimData( completion_data ):
   if 'detailed_info' in completion_data:
     vim_data[ 'info' ] = ToUtf8IfNeeded( completion_data[ 'detailed_info' ] )
     if doc_string:
-      vim_data[ 'info' ] += os.linesep + doc_string
+      vim_data[ 'info' ] += '\n' + doc_string
   elif doc_string:
     vim_data[ 'info' ] = doc_string
 
diff --git a/python/ycm/client/tests/completion_request_test.py b/python/ycm/client/tests/completion_request_test.py
index 1ca0a0c6..b23514ea 100644
--- a/python/ycm/client/tests/completion_request_test.py
+++ b/python/ycm/client/tests/completion_request_test.py
@@ -20,7 +20,6 @@
 from nose.tools import eq_
 from ycm.test_utils import MockVimModule
 vim_mock = MockVimModule()
-import os
 
 from .. import completion_request
 
@@ -57,7 +56,7 @@ class ConvertCompletionResponseToVimDatas_test:
       'abbr': 'MENU TEXT',
       'menu': 'EXTRA MENU INFO',
       'kind': 'k',
-      'info': 'DETAILED INFO' + os.linesep + 'DOC STRING',
+      'info': 'DETAILED INFO\nDOC STRING',
       'dup' : 1,
     } )
 
diff --git a/python/ycm/test_utils.py b/python/ycm/test_utils.py
index 1a7c15d6..f060466b 100644
--- a/python/ycm/test_utils.py
+++ b/python/ycm/test_utils.py
@@ -20,17 +20,44 @@
 from mock import MagicMock
 import sys
 
+# One-and only instance of mocked Vim object. The first 'import vim' that is
+# executed binds the vim module to the instance of MagicMock that is created,
+# and subsquent assignments to sys.modules[ 'vim' ] don't retrospectively update
+# them. The result is that while running the tests, we must assign only one
+# instance of MagicMock to sys.modules[ 'vim' ] and always return it.
+#
+# More explanation is available:
+# https://github.com/Valloric/YouCompleteMe/pull/1694
+VIM_MOCK = MagicMock()
+
 def MockVimModule():
   """The 'vim' module is something that is only present when running inside the
-  Vim Python interpreter, so we replace it with a MagicMock for tests. """
+  Vim Python interpreter, so we replace it with a MagicMock for tests. If you
+  need to add additional mocks to vim module functions, then use 'patch' from
+  mock module, to ensure that the state of the vim mock is returned before the
+  next test. That is:
+
+    from ycm.test_utils import MockVimModule
+    from mock import patch
+
+    # Do this once
+    MockVimModule()
+
+    @patch( 'vim.eval', return_value='test' )
+    @patch( 'vim.command', side_effect=ValueError )
+    def test( vim_command, vim_eval ):
+      # use vim.command via vim_command, e.g.:
+      vim_command.assert_has_calls( ... )
+
+  Failure to use this approach may lead to unexpected failures in other
+  tests."""
 
   def VimEval( value ):
     if value == "g:ycm_min_num_of_chars_for_completion":
       return 0
     return ''
 
-  vim_mock = MagicMock()
-  vim_mock.eval = MagicMock( side_effect = VimEval )
-  sys.modules[ 'vim' ] = vim_mock
-  return vim_mock
+  VIM_MOCK.eval = MagicMock( side_effect = VimEval )
+  sys.modules[ 'vim' ] = VIM_MOCK
 
+  return VIM_MOCK
diff --git a/python/ycm/tests/__init__.py b/python/ycm/tests/__init__.py
new file mode 100644
index 00000000..8b137891
--- /dev/null
+++ b/python/ycm/tests/__init__.py
@@ -0,0 +1 @@
+
diff --git a/python/ycm/tests/syntax_parse_test.py b/python/ycm/tests/syntax_parse_test.py
index 17e1184a..8692792b 100644
--- a/python/ycm/tests/syntax_parse_test.py
+++ b/python/ycm/tests/syntax_parse_test.py
@@ -17,11 +17,12 @@
 # You should have received a copy of the GNU General Public License
 # along with YouCompleteMe.  If not, see <http://www.gnu.org/licenses/>.
 
+from ycm.test_utils import MockVimModule
+MockVimModule()
+
 import os
 from nose.tools import eq_
 from hamcrest import assert_that, has_items
-from ycm.test_utils import MockVimModule
-vim_mock = MockVimModule()
 from ycm import syntax_parse
 
 
diff --git a/python/ycm/tests/vimsupport_test.py b/python/ycm/tests/vimsupport_test.py
index ff4e8e1c..936101a0 100644
--- a/python/ycm/tests/vimsupport_test.py
+++ b/python/ycm/tests/vimsupport_test.py
@@ -17,8 +17,12 @@
 # You should have received a copy of the GNU General Public License
 # along with YouCompleteMe.  If not, see <http://www.gnu.org/licenses/>.
 
+from ycm.test_utils import MockVimModule
+MockVimModule()
+
 from ycm import vimsupport
 from nose.tools import eq_
+from mock import MagicMock, call, patch
 
 
 def ReplaceChunk_SingleLine_Repl_1_test():
@@ -576,3 +580,88 @@ def _BuildChunk( start_line, start_column, end_line, end_column,
     },
     'replacement_text': replacement_text
   }
+
+
+def _Mock_tempname( arg ):
+  if arg == 'tempname()':
+    return '_TEMP_FILE_'
+
+  raise ValueError( 'Unexpected evaluation: ' + arg )
+
+
+@patch( 'vim.eval', side_effect=_Mock_tempname )
+@patch( 'vim.command' )
+@patch( 'vim.current' )
+def WriteToPreviewWindow_test( vim_current, vim_command, vim_eval ):
+  vim_current.window.options.__getitem__ = MagicMock( return_value = True )
+
+  vimsupport.WriteToPreviewWindow( "test" )
+
+  vim_command.assert_has_calls( [
+    call( 'silent! pclose!' ),
+    call( 'silent! pedit! _TEMP_FILE_' ),
+    call( 'silent! wincmd P' ),
+    call( 'silent! wincmd p' ) ] )
+
+  vim_current.buffer.__setitem__.assert_called_with(
+      slice( None, None, None ), [ 'test' ] )
+
+  vim_current.buffer.options.__setitem__.assert_has_calls( [
+    call( 'buftype', 'nofile' ),
+    call( 'swapfile', False ),
+    call( 'modifiable', False ),
+    call( 'modified', False ),
+    call( 'readonly', True ),
+  ], any_order = True )
+
+
+@patch( 'vim.eval', side_effect=_Mock_tempname )
+@patch( 'vim.current' )
+def WriteToPreviewWindow_MultiLine_test( vim_current, vim_eval ):
+  vim_current.window.options.__getitem__ = MagicMock( return_value = True )
+  vimsupport.WriteToPreviewWindow( "test\ntest2" )
+
+  vim_current.buffer.__setitem__.assert_called_with(
+      slice( None, None, None ), [ 'test', 'test2' ] )
+
+
+@patch( 'vim.eval', side_effect=_Mock_tempname )
+@patch( 'vim.command' )
+@patch( 'vim.current' )
+def WriteToPreviewWindow_JumpFail_test( vim_current, vim_command, vim_eval ):
+  vim_current.window.options.__getitem__ = MagicMock( return_value = False )
+
+  vimsupport.WriteToPreviewWindow( "test" )
+
+  vim_command.assert_has_calls( [
+    call( 'silent! pclose!' ),
+    call( 'silent! pedit! _TEMP_FILE_' ),
+    call( 'silent! wincmd P' ),
+    call( "echom 'test'" ),
+  ] )
+
+  vim_current.buffer.__setitem__.assert_not_called()
+  vim_current.buffer.options.__setitem__.assert_not_called()
+
+
+@patch( 'vim.eval', side_effect=_Mock_tempname )
+@patch( 'vim.command' )
+@patch( 'vim.current' )
+def WriteToPreviewWindow_JumpFail_MultiLine_test( vim_current,
+                                                  vim_command,
+                                                  vim_eval ):
+
+  vim_current.window.options.__getitem__ = MagicMock( return_value = False )
+
+  vimsupport.WriteToPreviewWindow( "test\ntest2" )
+
+  vim_command.assert_has_calls( [
+    call( 'silent! pclose!' ),
+    call( 'silent! pedit! _TEMP_FILE_' ),
+    call( 'silent! wincmd P' ),
+    call( "echom 'test'" ),
+    call( "echom 'test2'" ),
+  ] )
+
+  vim_current.buffer.__setitem__.assert_not_called()
+  vim_current.buffer.options.__setitem__.assert_not_called()
diff --git a/python/ycm/vimsupport.py b/python/ycm/vimsupport.py
index e00a12df..50f95d03 100644
--- a/python/ycm/vimsupport.py
+++ b/python/ycm/vimsupport.py
@@ -575,3 +575,65 @@ def SearchInCurrentBuffer( pattern ):
 
 def LineTextInCurrentBuffer( line ):
   return vim.current.buffer[ line ]
+
+
+def ClosePreviewWindow():
+  """ Close the preview window if it is present, otherwise do nothing """
+  vim.command( 'silent! pclose!' )
+
+
+def JumpToPreviewWindow():
+  """ Jump the vim cursor to the preview window, which must be active. Returns
+  boolean indicating if the cursor ended up in the preview window """
+  vim.command( 'silent! wincmd P' )
+  return vim.current.window.options[ 'previewwindow' ]
+
+
+def JumpToPreviousWindow():
+  """ Jump the vim cursor to its previous window position """
+  vim.command( 'silent! wincmd p' )
+
+
+def OpenFileInPreviewWindow( filename ):
+  """ Open the supplied filename in the preview window """
+  vim.command( 'silent! pedit! ' + filename )
+
+
+def WriteToPreviewWindow( message ):
+  """ Display the supplied message in the preview window """
+
+  # This isn't something that comes naturally to Vim. Vim only wants to show
+  # tags and/or actual files in the preview window, so we have to hack it a
+  # little bit. We generate a temporary file name and "open" that, then write
+  # the data to it. We make sure the buffer can't be edited or saved. Other
+  # approaches include simply opening a split, but we want to take advantage of
+  # the existing Vim options for preview window height, etc.
+
+  ClosePreviewWindow()
+
+  OpenFileInPreviewWindow( vim.eval( 'tempname()' ) )
+
+  if JumpToPreviewWindow():
+    # We actually got to the preview window. By default the preview window can't
+    # be changed, so we make it writable, write to it, then make it read only
+    # again.
+    vim.current.buffer.options[ 'modifiable' ] = True
+    vim.current.buffer.options[ 'readonly' ]   = False
+
+    vim.current.buffer[:] = message.splitlines()
+
+    vim.current.buffer.options[ 'buftype' ]    = 'nofile'
+    vim.current.buffer.options[ 'swapfile' ]   = False
+    vim.current.buffer.options[ 'modifiable' ] = False
+    vim.current.buffer.options[ 'readonly' ]   = True
+
+    # We need to prevent closing the window causing a warning about unsaved
+    # file, so we pretend to Vim that the buffer has not been changed.
+    vim.current.buffer.options[ 'modified' ]   = False
+
+    JumpToPreviousWindow()
+  else:
+    # We couldn't get to the preview window, but we still want to give the user
+    # the information we have. The only remaining option is to echo to the
+    # status area.
+    EchoText( message )