strange behaviour of SwCrsrShell::aVisArea

Hi all,

I'm trying to enhance the patch for fdo#40465 as the patch results in behaviour that in some cases 
is a regression.
When zooming I want to know if the cursor is visible.
The function SwCrsrShell::CrsrIsVisible() some times returns sal_False when the cursor absolutely 
is visible in the view.

I collected some traces of the cursor position and of the visible area (SwCrsrShell::aVisArea) and 
to my amazement the area differs between two (visually) identical views.

I opened a writer document with 22 pages, scrolled to page 16, with cursor at page 16.
SwCrsrShell::CrsrIsVisible() returns sal_True, zoom % is 60, SwCrsrShell::aVisArea.Top(), Bottom() 
give 170460 and 188635.
I then move the cursor one line down and back (no scrolling), zoom to 55% and back to 60%.
The visible area looks identical, the cursor is back in the same location.
Now SwCrsrShell::CrsrIsVisible() returns sal_True, zoom % is 60, SwCrsrShell::aVisArea.Top(), 
Bottom() give 115860 and 134035. The cursor rectangle position has not changed and now is not in 
SwCrsrShell::aVisArea (cursor top/bottom is 121272/122191).

What is the explanation for this behaviour?
If this is 'normal', how can I (jn the code) find out if the cursor is visible?

If required, I can upload the document as well as the code for the trace into bug fdo#40465.



Winfried