Ticket #3580: 3580.patch

_source/plugins/undo/plugin.js

@@ -55 +55 @@
                         function recordCommand( event )
                         {
                                 // If the command hasn't been marked to not support undo.
-                                if ( undoManager.enabled && event.data.command.canUndo !== false )
+                                var command = event.data.command;
+                                if ( undoManager.enabled && command.canUndo !== false &&
+                                                !( command.canUndoAfter === false && event.name == 'afterCommandExec' ) )
                                         undoManager.save();
                         }
 

_source/plugins/sourcearea/plugin.js

@@ -188 +188 @@
                         {
                                 editor.getCommand( 'source' ).setState( CKEDITOR.TRISTATE_DISABLED );
                                 editor.setMode( editor.mode == 'source' ? 'wysiwyg' : 'source' );
-                        }
+                        },
+
+                        /*
+                         * #3580: editor.mode is empty right after executing this command, which
+                         * causes editor.getSnapshot() to return false. Grabbing one more snapshot
+                         * after command execution is meaningless for Source anyway.
+                         *
+                         * So disable it.
+                         */
+                        canUndoAfter : false
                 }
         }
 };