Avoid empty space above the top of the document

Avoid empty space above the top of the document

If the height of the first row of the document has become smaller (for
example when it contains an image which size has shrunk), that the
anchor offset has to be adapted so that there is not a large blank
space at the top of the document.

(cherry picked from commit d6aecbda0f5929ab04805ac6e19cd48f3c4a059b)