I continued to investigate...
Nothing changed when :
- I tried the patch
- I switched from 64bits to 32bits kernel.
- I switched for an usb mouse
- I tried on another completely different computer with a totally different mouse
- A friend saw the defect and reproduced it on Cygwin (!!) with Xfree86
So it appears that it is not a problem of linux or mouse related pb.
It seems that it is just X11 that doesn't handle that.
Any idea from where it comes from internally ? A late buffer flush when the mouse is clicked ? How it is done in windows ? What misses in X11 ?
You cannot quickly select an url in a browser, move a window or select a series of objects without your mouse clicking centimeters away the point you clicked.
I just wonder... How people can stand this all day long ?