DBE on nv, Shape and XAA questions

* Is it correct, that the "nv" driver does not support DBE (double buffer extension)?


* How fast are "Screen to screen bit blits" ?

I tuned sawfish WM, so that when resizing windows it sets the border window (WM
decoration) to a suitable window gravity, so it's up to the X server to move it
correctly (WM does not redraw). I avoided shaped windows.

I use the "mga" driver, so it has "Screen to screen bit blits" XXA call.

Yet, if I resize (in 1 direction) continuously the window, i see the vertical
line as a staircase line:


   |        |
b  |        |
l  |       |
a  | ->    |
c  |       |
k  |      |
   |      |

Any idea if it can be improved? 



* Shape vs. flickering

if i run xlogo -shape (resize it to make things clear) and move the logo window, i see
flickering (the window below is obscured and redrawn even outside of the logo).

I would likt to know, if "Screen to Screen color expansion" XAA call could be used to
avoid it.