Вот что написали троли по этому поводу:
I am able to reproduce this problem when using the Qt4.2.3 release but
this problem is already resolved for the Qt4.3 release due to the new
workspace that is introduced for this release, the QMdiArea class.
Please try the latest Qt4.3 snapshots and if you still get the same
problematic behavior then please get back to us.
I used the mdi example that is provided with the Qt package ( located
under $QTDIR/examples/mainwindows/ directory ) and it seems that the
pos() method returns different values everytime the subwindow is moved
withing the workspace area.