Created attachment 124692 [details]
screenshot

Created attachment 124693 [details]
X config as used by anaconda

Need to see the X log also, to see what modelines are being rejected by the
X server, and see what choices it is making to determine the final physical
and vertical resolution.

Is this occuring only in anaconda, or does it occur on a startx invocation
on a fully installed system also?

I completed the text mode install, which set up X correctly.
(Though I ended up using my old stored X config so that dual head worked).

I'll boot an installer again, and grab you an X log.
iirc though, it was very sparse on details.

lots has changed since this was reported.  It's best to try a newer install
from rawhide.