diff --git a/python/ycm/tests/client/command_request_test.py b/python/ycm/tests/client/command_request_test.py
index 6c783c0a..a205990d 100644
--- a/python/ycm/tests/client/command_request_test.py
+++ b/python/ycm/tests/client/command_request_test.py
@@ -267,10 +267,12 @@ class Response_Detection_test( object ):
       # GoToResponse_QuickFix_test, so here we just check that the right call is
       # made
       with patch( 'ycm.vimsupport.SetQuickFixList' ) as set_qf_list:
-        request = CommandRequest( [ command ] )
-        request._response = response
-        request.RunPostCommandActionsIfNeeded()
-        ok_( set_qf_list.called )
+        with patch( 'ycm.vimsupport.OpenQuickFixList' ) as open_qf_list:
+          request = CommandRequest( [ command ] )
+          request._response = response
+          request.RunPostCommandActionsIfNeeded()
+          ok_( set_qf_list.called )
+          ok_( open_qf_list.called )
 
     basic_goto = {
       'filepath': 'test',
diff --git a/python/ycm/tests/vimsupport_test.py b/python/ycm/tests/vimsupport_test.py
index 3009d9f9..db8dcc81 100644
--- a/python/ycm/tests/vimsupport_test.py
+++ b/python/ycm/tests/vimsupport_test.py
@@ -27,7 +27,8 @@ from builtins import *  # noqa
 
 from ycm.tests import PathToTestFile
 from ycm.tests.test_utils import ( CurrentWorkingDirectory, ExtendedMock,
-                                   MockVimCommand, MockVimModule, VimBuffer )
+                                   MockVimBuffers, MockVimCommand,
+                                   MockVimModule, VimBuffer )
 MockVimModule()
 
 from ycm import vimsupport
@@ -39,6 +40,47 @@ import os
 import json
 
 
+@patch( 'vim.eval', new_callable = ExtendedMock )
+def SetLocationList_test( vim_eval ):
+  diagnostics = [ {
+    'bufnr': 3,
+    'filename': 'some_filename',
+    'lnum': 5,
+    'col': 22,
+    'type': 'E',
+    'valid': 1
+  } ]
+  vimsupport.SetLocationList( diagnostics )
+  vim_eval.assert_called_once_with(
+    'setloclist( 0, {0} )'.format( json.dumps( diagnostics ) ) )
+
+
+@patch( 'ycm.vimsupport.VariableExists', return_value = True )
+@patch( 'ycm.vimsupport.SetFittingHeightForCurrentWindow' )
+@patch( 'vim.command', new_callable = ExtendedMock )
+def OpenLocationList_test( vim_command, fitting_height, variable_exists ):
+  vimsupport.OpenLocationList( focus = False, autoclose = True )
+  vim_command.assert_has_exact_calls( [
+    call( 'botright lopen' ),
+    call( 'au WinLeave <buffer> q' ),
+    call( 'doautocmd User YcmLocationOpened' ),
+    call( 'silent! wincmd p' )
+  ] )
+  fitting_height.assert_called_once_with()
+  variable_exists.assert_called_once_with( '#User#YcmLocationOpened' )
+
+
+@patch( 'ycm.vimsupport.GetIntValue', return_value = 120 )
+@patch( 'vim.command' )
+def SetFittingHeightForCurrentWindow_test( vim_command, *args ):
+  # Create a buffer with one line that is longer than the window width.
+  current_buffer = VimBuffer( 'buffer',
+                              contents = [ 'a' * 140 ] )
+  with MockVimBuffers( [ current_buffer ], current_buffer ):
+    vimsupport.SetFittingHeightForCurrentWindow()
+  vim_command.assert_called_once_with( '2wincmd _' )
+
+
 def AssertBuffersAreEqualAsBytes( result_buffer, expected_buffer ):
   eq_( len( result_buffer ), len( expected_buffer ) )
   for result_line, expected_line in zip( result_buffer, expected_buffer ):
diff --git a/python/ycm/tests/youcompleteme_test.py b/python/ycm/tests/youcompleteme_test.py
index e7747951..27990bc5 100644
--- a/python/ycm/tests/youcompleteme_test.py
+++ b/python/ycm/tests/youcompleteme_test.py
@@ -295,3 +295,122 @@ def YouCompleteMe_ShowDetailedDiagnostic_MessageFromServer_test(
   post_vim_message.assert_has_exact_calls( [
     call( 'some_detailed_diagnostic', warning = False )
   ] )
+
+
+@YouCompleteMeInstance()
+@patch( 'ycm.vimsupport.PostVimMessage', new_callable = ExtendedMock )
+def YouCompleteMe_ShowDiagnostics_FiletypeNotSupported_test( ycm,
+                                                             post_vim_message ):
+
+  current_buffer = VimBuffer( 'buffer', filetype = 'not_supported' )
+  with MockVimBuffers( [ current_buffer ], current_buffer ):
+    ycm.ShowDiagnostics()
+
+  post_vim_message.assert_called_once_with(
+    'Native filetype completion not supported for current file, '
+    'cannot force recompilation.', warning = False )
+
+
+@YouCompleteMeInstance()
+@patch( 'ycm.youcompleteme.YouCompleteMe.FiletypeCompleterExistsForFiletype',
+        return_value = True )
+@patch( 'ycm.vimsupport.PostVimMessage', new_callable = ExtendedMock )
+@patch( 'ycm.vimsupport.SetLocationList', new_callable = ExtendedMock )
+def YouCompleteMe_ShowDiagnostics_NoDiagnosticsDetected_test(
+  ycm, set_location_list, post_vim_message, *args ):
+
+  current_buffer = VimBuffer( 'buffer', filetype = 'cpp' )
+  with MockVimBuffers( [ current_buffer ], current_buffer ):
+    with patch( 'ycm.client.event_notification.EventNotification.Response',
+                return_value = {} ):
+      ycm.ShowDiagnostics()
+
+  post_vim_message.assert_has_exact_calls( [
+    call( 'Forcing compilation, this will block Vim until done.',
+          warning = False ),
+    call( 'Diagnostics refreshed', warning = False ),
+    call( 'No warnings or errors detected.', warning = False )
+  ] )
+  set_location_list.assert_called_once_with( [] )
+
+
+@YouCompleteMeInstance( { 'log_level': 'debug',
+                          'keep_logfiles': 1,
+                          'open_loclist_on_ycm_diags': 0 } )
+@patch( 'ycm.youcompleteme.YouCompleteMe.FiletypeCompleterExistsForFiletype',
+        return_value = True )
+@patch( 'ycm.vimsupport.PostVimMessage', new_callable = ExtendedMock )
+@patch( 'ycm.vimsupport.SetLocationList', new_callable = ExtendedMock )
+def YouCompleteMe_ShowDiagnostics_DiagnosticsFound_DoNotOpenLocationList_test(
+  ycm, set_location_list, post_vim_message, *args ):
+
+  diagnostic = {
+    'kind': 'ERROR',
+    'text': 'error text',
+    'location': {
+      'filepath': 'buffer',
+      'column_num': 2,
+      'line_num': 19
+    }
+  }
+
+  current_buffer = VimBuffer( 'buffer', filetype = 'cpp', number = 3 )
+  with MockVimBuffers( [ current_buffer ], current_buffer ):
+    with patch( 'ycm.client.event_notification.EventNotification.Response',
+                return_value = [ diagnostic ] ):
+      ycm.ShowDiagnostics()
+
+  post_vim_message.assert_has_exact_calls( [
+    call( 'Forcing compilation, this will block Vim until done.',
+          warning = False ),
+    call( 'Diagnostics refreshed', warning = False )
+  ] )
+  set_location_list.assert_called_once_with( [ {
+      'bufnr': 3,
+      'lnum': 19,
+      'col': 2,
+      'text': 'error text',
+      'type': 'E',
+      'valid': 1
+  } ] )
+
+
+@YouCompleteMeInstance( { 'open_loclist_on_ycm_diags': 1 } )
+@patch( 'ycm.youcompleteme.YouCompleteMe.FiletypeCompleterExistsForFiletype',
+        return_value = True )
+@patch( 'ycm.vimsupport.PostVimMessage', new_callable = ExtendedMock )
+@patch( 'ycm.vimsupport.SetLocationList', new_callable = ExtendedMock )
+@patch( 'ycm.vimsupport.OpenLocationList', new_callable = ExtendedMock )
+def YouCompleteMe_ShowDiagnostics_DiagnosticsFound_OpenLocationList_test(
+  ycm, open_location_list, set_location_list, post_vim_message, *args ):
+
+  diagnostic = {
+    'kind': 'ERROR',
+    'text': 'error text',
+    'location': {
+      'filepath': 'buffer',
+      'column_num': 2,
+      'line_num': 19
+    }
+  }
+
+  current_buffer = VimBuffer( 'buffer', filetype = 'cpp', number = 3 )
+  with MockVimBuffers( [ current_buffer ], current_buffer ):
+    with patch( 'ycm.client.event_notification.EventNotification.Response',
+                return_value = [ diagnostic ] ):
+      ycm.ShowDiagnostics()
+
+  post_vim_message.assert_has_exact_calls( [
+    call( 'Forcing compilation, this will block Vim until done.',
+          warning = False ),
+    call( 'Diagnostics refreshed', warning = False )
+  ] )
+  set_location_list.assert_called_once_with( [ {
+      'bufnr': 3,
+      'lnum': 19,
+      'col': 2,
+      'text': 'error text',
+      'type': 'E',
+      'valid': 1
+  } ] )
+  open_location_list.assert_called_once_with( focus = True )