The undo command behaves incorrectly for text that has been pasted from the clipboard over other text.

Copy some text to the clipboard.
Select some text in Bluefish.
Do paste.

The original selected text is overwritten with the clipboard text.

Now do Undo.

Expected behaviour: pasted text vanishes, original reappears.
Actual behaviour: both pieces of text are shown side by side. This is incorrect, as it is a state the document was never in.