[Enkel]

I'm sure this question has been asked before but I can't find an answer in a
search.
When I try to reformat a selection of text (for example to bold it or
centre it on a line), the whole document reformats. However when I click on
the "Undo" button the rest of the document reverts to what it was, leaving my
selected text reformatted. This is annoying & I'm sure I've seen the problem
before and seen a fix. But I can't remember what it was. Any help, please?