After upgrading to Squeeze I noticed that the "downloads finished" popup
window shown by Iceweasel leaves strange artifacts at the place it is
shown (bottom-right corner of the screen, where a "pager" in a typical
GNOME setup is located) and then made gradually slide downwards.
Please see the attached screenshot.

Surely, it looks like a problem with X graphics driver, but I see no
graphics-related problems at all with any of installed packages,
that is, only this particular iceweasel window leaves that strange
garbage.

Oddly enough, there seems to be no way of removing these artifacts by
moving other windows over that place etc; this does not cause updating
of the underlying screen area.

If you feel like this is an issue with X, feel free to reassign the bug
to them; I'll provide more info, if needed.

I have this video card:
~% lspci |grep -i graphic
00:02.0 VGA compatible controller: Intel Corporation Mobile 4 Series
Chipset Integrated Graphics Controller (rev 07)
00:02.1 Display controller: Intel Corporation Mobile 4 Series Chipset
Integrated Graphics Controller (rev 07)

And X.org packages installed on my system are:
ii  xserver-xorg                                    1:7.5+7
ii  xserver-xorg-core                               2:1.7.7-6
ii  xserver-xorg-input-all                          1:7.5+7
ii  xserver-xorg-input-evdev                        1:2.3.2-6
ii  xserver-xorg-input-kbd                          1:1.4.0-2
ii  xserver-xorg-input-mouse                        1:1.5.0-2
ii  xserver-xorg-input-synaptics                    1.2.2-2
ii  xserver-xorg-input-wacom                        0.10.5+20100416-1
ii  xserver-xorg-video-intel                        2:2.9.1-4