Update - using current Mesa and agd5f drm modules on 22.214.171.124 on rv670 AGP card.
Hello is better - it doesn't crash anymore when part off screen and resizing on screen is OK.
Top and left clipping still gives distorted image, which gets redrawn correctly when returning on screen. Sometimes the return to screen results in black becoming grey or rarely whole window in white. In both cases forcing a redraw by resizing fixes the output.
Edit: I also notice that it will draw over the top of and not clean up an xterm if obscured by it.
glxgears runs @ 10fps, but does not draw anything.
Edit: I see on http://jbridgman.livejournal.com/ that r6xx colour problems are known, so I guess I get black - it's the same on the "working on r7xx" redbook progs listed there that use colour.