Created attachment 3469 
Proposed patch

Hi,

It seems there's a typo in docs/manual/Makefile.am: before installing the html files, a test is made to ensure that they exist, but "test -d" is used instead of "test -f", so it is never true, and files are never installed. The attached patch fixes this issue here.

Also, I don't know if it's related, but when I build xfce4-panel twice in a row, all html and css files in docs/manual are deleted, so the second time, the doc isn't installed…

Thanks!