When editing a file, the last line in the file doesn't get a newline, when this is not typed in. Editors like VIM do insert such a newline if it is not there. While one could argue that this is not a bug and in fact is the way it should be, I think it would be better to follow other editors like VI.
Closing. There is no reason for a text editor to add a new line at the end of a document. IDE's do this, but mousepad is not an IDE, is's a simple text editor.