Writer bug: Different layout model with same document model?

Hi developers!

The patch I wrote in the Hackfest Hamburg seems to cause trouble. It 
introduced a regression, see
https://bugs.documentfoundation.org/show_bug.cgi?id=117187

What happens: I wrote a patch which considers upper space above a 
paragraph for intersecting with fly objects (I simply increased the 
height of the rectangle, commit
d07fc485d46f431405a3f6a002f951a08c559677 )
With that, an annoying bug was fixed. But now, strange regressions 
appear (see bug 117187):

After initially opening the odf file in Writer, the paragraph with the 
date is too low. If I move the text box located above the date to the 
left, the date jumps in the correct position. If I hit undo, everything 
is then OK (?)...
So there is a difference in layout between initial opening and the 
situation after undo (which should be the same by definition...). 
Strange because the nodes.xml is the same before and after the undo action.

I thought layout is based on the document model. How can it be different 
in this situation?

Have a nice day!
Patrick