Thank you Glenn.

>Very strange; I have no idea what might cause that.

I also feel it strange.
and that is critical setting to see the bug.

>Maybe post the details of your
>video card and driver to see if it means anything to anyone.

this is details:

----graphic card----
ATI Radeon HD 5700 Series
memory: GDDR5 1200Mhz 1024MB 76.8GB/s
BUS: PCI Express 2.0 x16 850MHz
driver-version 8.723.1-00408a-098580C-ATI
2D driver-vesion: 8.72.11
Catalyst Control Center version: 2.12

----cpu info----
core i5 cpu 760 @ 2.80GHz

----ubuntu (lucid)---
wubi 32bit
Linux ubuntu 2.6.32-28-generic #55-Ubuntu SMP Mon Jan 10 23:42:43 UTC 2011
x86_64 GNU/Linux