Mouse position is wrong on some desktop

[hjuvi]

Hi,

On mouse clicks, I want to go to the line on which mouse was clicked.
On left click, this is easy : current line is updated.
On right click, I have to use this : (move-to-window-line (cdr (last
(mouse-position))))

It worked fine for a long time... I had noticed that the line was
wrong (one line shift) on Windows, but I stopped supporting Windows
(for many other reasons ;))

My problem is that today, someone running on KDE desktop is using my
code, and the line is wrong (one line shift, the same as with
Windows). I run on Gnome desktop.
I'm not sure this is particularly due to KDE (it used to work on KDE).

Any idea, why mouse-position does not return correct line, depending
on the desktop (or maybe something else...) ?

Thanks for your help...