msi installer window can not be moved during "Computing space requirements" (e.g. Python 3.4.4)

0.4.10-RC-30-g51a70dc gcc dbg lin, VBox 4.3.12, VBEMP
I think we can use any arbitrary msi installer for that, but "Python 3.4.4" from rapps is where I report against, because it computes for a long time which simplifies debugging.

Observed result
While the msi setup does "Computing space requirements" the window of the installer can not be moved/dragged at the titlebar. After these computations have finished, the dragging is possible again.
0.4.10-RC-30-g51a70dc__csr.png
0.4.10-RC-30-g51a70dc__csr.log is useless

Expected result
WinXP SP3 allows to drag/move the window at the title-bar when these computations do take place.