I think it is tied to the recent processor, rather than to the radeon (but might be wrong).
I have encountered a number of similar issues and it has been a very painful process to solve (and i do not remember the detailed process you will have to do some edit and test around).
That was with 4.0; you have not said if your problem is with 4.0 or cooking.
It took me a long time to solve all the issues. And quite frankly it has been such a pain that I went away from slitaz at that time, almost completely.
From what I remember right click in the background which bring up a menu and allows to start a terminal from openbox where you can start lxpanel, which gives the menu.
I think there is another command to run to start whatever allows to move the windows around (and i do not remember what i did to make it run, maybe run lxsession manually or something similar, I am really no expert on this)
Start there and see if it helps.
I have been reading a lot at that time on the Arch wiki and it helped me a lot, I found it to be very well documented; you may want to read the lxde page. The arch wiki provides very well documented step by step wikis.
Unless any of the very knowledgeable people of this forum has a better advice for you, of course.
I think I had to edit some X file to solve keyboard mapping issues but i am not sure.
Not clear you will need to do what is below (until the bottom of the post), I had to:
Also i had to modify the boot line to include something like this (this is from usb so not grub, adjust per your configuration if it helps)
append initrd=/boot/rootfs.gz rw root=/dev/null vga=normal nomodeset autologin home=ae10e45a-518b-404c-b533-636cd52e8f89 screen=text lang=fr_FR
this goes to a console where i have to login as root then run slim manually.
Hope this helps. I'll post again if I find what I did.
Apologies if what I posted here is trivial to you.