How to stop edge of Plasma::Dialog to Change

[Weng Xuetian]

Hi Plasma world,
Plasma dialog will be extend to the edge of panel or screen, this is a 
good feature for popup window. But not suitable for my case. Is there 
any way to stop this?

Currently I need a window themed by plasma and keep it on the top of 
screen, Plasma::Dialog seems to be suitable with some window attribute 
set, in spite of the edge changing, because this window will move and 
resize frequently. Or should I use some other class?

Regards,
Xuetian