diff --git a/python3/vimspector/output.py b/python3/vimspector/output.py
index c74fe3e..cb91e51 100644
--- a/python3/vimspector/output.py
+++ b/python3/vimspector/output.py
@@ -32,7 +32,8 @@ class OutputView( object ):
       self._CreateBuffer( category )
 
     with utils.ModifiableScratchBuffer( self._buffers[ category ] ):
-      self._buffers[ category ].append( event[ 'output' ].splitlines() )
+      utils.AppendToBuffer( self._buffers[ category ],
+                            event[ 'output' ].splitlines() )
 
   def Reset( self ):
     self.Clear()
@@ -54,7 +55,8 @@ class OutputView( object ):
       with utils.RestoreCurrentBuffer( self._window ):
         vim.command( 'enew' )
         self._buffers[ category ] = vim.current.buffer
-        self._buffers[ category ].append( category + '-----' )
+        utils.AppendToBuffer( self._buffers[ category ],
+                              category + '-----' )
 
         utils.SetUpHiddenBuffer( self._buffers[ category ],
                                  'vimspector.Output:{0}'.format( category ) )
diff --git a/python3/vimspector/stack_trace.py b/python3/vimspector/stack_trace.py
index ae65db4..a71cef9 100644
--- a/python3/vimspector/stack_trace.py
+++ b/python3/vimspector/stack_trace.py
@@ -93,8 +93,11 @@ class StackTraceView( object ):
       for thread in self._threads:
         icon = '+' if '_frames' not in thread else '-'
 
-        self._buf.append( '{0} Thread: {1}'.format( icon, thread[ 'name' ] ) )
-        self._line_to_thread[ len( self._buf ) ] = thread
+        line = utils.AppendToBuffer(
+          self._buf,
+          '{0} Thread: {1}'.format( icon, thread[ 'name' ] ) )
+
+        self._line_to_thread[ line ] = thread
 
         self._DrawStackTrace( thread )
 
@@ -193,9 +196,10 @@ class StackTraceView( object ):
       else:
         source = { 'name': '<unknown>' }
 
-      self._buf.append(
+      line = utils.AppendToBuffer(
+        self._buf,
         '  {0}: {1}@{2}:{3}'.format( frame[ 'id' ],
                                      frame[ 'name' ],
                                      source[ 'name' ],
                                      frame[ 'line' ] ) )
-      self._line_to_frame[ len( self._buf ) ] = frame
+      self._line_to_frame[ line ] = frame
diff --git a/python3/vimspector/utils.py b/python3/vimspector/utils.py
index 93f2a94..e102063 100644
--- a/python3/vimspector/utils.py
+++ b/python3/vimspector/utils.py
@@ -24,6 +24,7 @@ _log_handler = logging.FileHandler( os.path.expanduser( '~/.vimspector.log' ) )
 _log_handler.setFormatter(
     logging.Formatter( '%(asctime)s - %(levelname)s - %(message)s' ) )
 
+
 def SetUpLogging( logger ):
   logger.setLevel( logging.DEBUG )
   if _log_handler not in logger.handlers:
@@ -171,3 +172,20 @@ def SelectFromList( prompt, options ):
 def AskForInput( prompt ):
   with InputSave():
     return vim.eval( "input( '{0}' )".format( Escape( prompt ) ) )
+
+
+def AppendToBuffer( buf, line_or_lines ):
+  # After clearing the buffer (using buf[:] = None) there is always a single
+  # empty line in the buffer object and no "is empty" method.
+  if len( buf ) > 1 or buf[ 0 ]:
+    line = len( buf ) + 1
+    buf.append( line_or_lines )
+  elif isinstance( line_or_lines, str ):
+    line = 1
+    buf[-1] = line_or_lines
+  else:
+    line = 1
+    buf[:] = line_or_lines
+
+  # Return the first Vim line number (1-based) that we just set.
+  return line
diff --git a/python3/vimspector/variables.py b/python3/vimspector/variables.py
index bc88cf8..2ed284d 100644
--- a/python3/vimspector/variables.py
+++ b/python3/vimspector/variables.py
@@ -21,6 +21,7 @@ from vimspector import utils
 
 View = namedtuple( 'View', [ 'win', 'lines', 'draw' ] )
 
+
 class VariablesView( object ):
   def __init__( self, connection, variables_win, watches_win ):
     self._vars = View( variables_win, {}, self._DrawScopes )
@@ -190,8 +191,8 @@ class VariablesView( object ):
 
   def _DrawVariables( self, view,  variables, indent ):
     for variable in variables:
-      view.lines[ len( view.win.buffer ) + 1 ] = variable
-      view.win.buffer.append(
+      line = utils.AppendToBuffer(
+        view.win.buffer,
         '{indent}{icon} {name} ({type_}): {value}'.format(
           indent = ' ' * indent,
           icon = '+' if ( variable[ 'variablesReference' ] > 0 and
@@ -199,6 +200,7 @@ class VariablesView( object ):
           name = variable[ 'name' ],
           type_ = variable.get( 'type', '<unknown type>' ),
           value = variable.get( 'value', '<unknown value>' ) ).split( '\n' ) )
+      view.lines[ line ] = variable
 
       if '_variables' in variable:
         self._DrawVariables( view, variable[ '_variables' ], indent + 2 )
@@ -216,22 +218,22 @@ class VariablesView( object ):
     with utils.RestoreCursorPosition():
       with utils.ModifiableScratchBuffer( self._watch.win.buffer ):
         self._watch.win.buffer[:] = None
-        self._watch.win.buffer.append( 'Watches: ----' )
+        utils.AppendToBuffer( self._watch.win.buffer, 'Watches: ----' )
         for watch in self._watches:
-          self._watch.win.buffer.append(
-            'Expression: ' + watch[ 'expression' ] )
-          watch[ '_line' ] = len( self._watch.win.buffer )
+          line = utils.AppendToBuffer( self._watch.win.buffer,
+                                       'Expression: ' + watch[ 'expression' ] )
+          watch[ '_line' ] = line
           self._DrawWatchResult( 2, watch )
 
   def _DrawScope( self, indent, scope ):
     icon = '+' if ( scope[ 'variablesReference' ] > 0 and
                     '_variables' not in scope ) else '-'
 
-    self._vars.lines[ len( self._vars.win.buffer ) + 1 ] = scope
-    self._vars.win.buffer.append( '{0}{1} Scope: {2}'.format(
-      ' ' * indent,
-      icon,
-      scope[ 'name' ] ) )
+    line = utils.AppendToBuffer( self._vars.win.buffer,
+                                 '{0}{1} Scope: {2}'.format( ' ' * indent,
+                                                             icon,
+                                                             scope[ 'name' ] ) )
+    self._vars.lines[ line ] = scope
 
     if '_variables' in scope:
       indent += 2
@@ -242,7 +244,6 @@ class VariablesView( object ):
       return
 
     result = watch[ '_result' ]
-    self._watch.lines[ len( self._watch.win.buffer ) + 1 ] = result
 
     icon = '+' if ( result[ 'variablesReference' ] > 0 and
                     '_variables' not in result ) else '-'
@@ -250,7 +251,9 @@ class VariablesView( object ):
     line =  '{0}{1} Result: {2} '.format( ' ' * indent,
                                           icon,
                                           result[ 'result' ] )
-    self._watch.win.buffer.append( line.split( '\n' ) )
+    line = utils.AppendToBuffer( self._watch.win.buffer,
+                                 line.split( '\n' ) )
+    self._watch.lines[ line ] = result
 
     if '_variables' in result:
       indent = 4