xrandr and radeon drives BenQ FP231W incorrectly

Just upgraded to gutsy today. I have a BenQ FP231W connected to my laptop's Radeon 9200. I try to turn off the laptop panel and use only the FP231W:

  xrandr --output LVDS --off --output VGA-0 --auto

This does switch off the laptop panel and enable the external display, but the picture on the external display is offset about 5cm to the right, and so is missing 5cm off the right hand side of the monitor. Looking in the FP231W OSD menu, it shows that the monitor thinks it is being run at 1600x1200, even though the computer thinks it is running at 1920x1200.

If I tell xrandr to run at 1600x1200 I get the reverse effect: the computer is acting as though the display is 1600x1200, but the display thinks it is running at 1920x1200, and so the image is stretched horizontally.

I have found that I can work around this by injecting the correct modeline/resolution combination into xrandr like so:

toojays@fuzz:~$ xrandr --newmode native 160.9 1920 1980 2040 2160 1200 1201 1204 1242
toojays@fuzz:~$ xrandr --addmode VGA-0 native
toojays@fuzz:~$ xrandr --output LVDS --off --output VGA-0 --mode native

The strange thing is that the modeline parameters I inject are the same ones which xrandr already seems to have detected. It looks as if the default resolution and timing params are getting scrambled somehow in xrandr or the radeon driver.

toojays@fuzz:~$ xrandr --verbose -q
Screen 0: minimum 320 x 200, current 1920 x 1200, maximum 1920 x 1200
LVDS connected (normal left inverted right)
        Identifier: 0x4c
        Timestamp: 2036037239
        Subpixel: horizontal rgb
        Clones:
        CRTCs: 0
        EDID_DATA:
                00ffffffffffff0006102a9c00000000
                000c0101801812780a6dc08f554e8927
                22505400080001010101010101010101
                01010101010164190040410026301888
                3600f5b8000000180000000100061020
                00000000000000000a20000000fe004c
                503132315830342d43320a20000000fc
                00434f4c4f52204c43440a2020200025
                scaler: full
        backlight: 0 (0x00000000) range: (0,255)
  1024x768 (0x4f) 65.0MHz -HSync -VSync
        h: width 1024 start 1048 end 1184 total 1344 skew 0 clock 48.4KHz
        v: height 768 start 771 end 777 total 806 clock 60.0Hz
  800x600 (0x50) 40.0MHz +HSync +VSync
        h: width 800 start 840 end 968 total 1056 skew 0 clock 37.9KHz
        v: height 600 start 601 end 605 total 628 clock 60.3Hz
  640x480 (0x51) 25.2MHz -HSync -VSync
        h: width 640 start 656 end 752 total 800 skew 0 clock 31.5KHz
        v: height 480 start 490 end 492 total 525 clock 59.9Hz
VGA-0 connected 1920x1200+0+0 (0x6d) normal (normal left inverted right) 496mm x 310mm
        Identifier: 0x4d
        Timestamp: 2036037239
        Subpixel: no subpixels
        Clones:
        CRTC: 1
        CRTCs: 0 1
        EDID_DATA:
                00ffffffffffff0009d169760d000000
                1f0e01036c321f78eacf60a355499925
                105054bdef80714f8180818c818fa940
                010101010101d73e80f070b02a403c3c
                1300f0361100001e000000fc0042656e
                51204650323331570a20d73e80f07038
                9a403c3c1300f0361100001e000000fd
                00384c1f5010000a2020202020200046
        load_detection: 0 (0x00000000) range: (0,1)
  1920x1200 (0x52) 160.9MHz +HSync +VSync
        h: width 1920 start 1980 end 2040 total 2160 skew 0 clock 74.5KHz
        v: height 1200 start 1201 end 1204 total 1242 clock 60.0Hz
  1920x1080 (0x53) 160.9MHz +HSync +VSync
        h: width 1920 start 1980 end 2040 total 2160 skew 0 clock 74.5KHz
        v: height 1080 start 1081 end 1084 total 1234 clock 60.4Hz
  1600x1200 (0x54) 161.0MHz -HSync +VSync
        h: width 1600 start 1712 end 1880 total 2160 skew 0 clock 74.5KHz
        v: height 1200 start 1203 end 1207 total 1245 clock 59.9Hz
  1280x1024 (0x55) 138.8MHz -HSync +VSync
        h: width 1280 start 1368 end 1504 total 1728 skew 0 clock 80.3KHz
        v: height 1024 start 1027 end 1034 total 1072 clock 74.9Hz
  1280x1024 (0x56) 135.0MHz +HSync +VSync
        h: width 1280 start 1296 end 1440 total 1688 skew 0 clock 80.0KHz
        v: height 1024 start 1025 end 1028 total 1066 clock 75.0Hz
  1280x1024 (0x57) 133.0MHz -HSync +VSync
        h: width 1280 start 1368 end 1504 total 1728 skew 0 clock 77.0KHz
        v: height 1024 start 1027 end 1034 total 1070 clock 71.9Hz
  1280x1024 (0x58) 109.0MHz -HSync +VSync
        h: width 1280 start 1368 end 1496 total 1712 skew 0 clock 63.7KHz
        v: height 1024 start 1027 end 1034 total 1063 clock 59.9Hz
  1152x864 (0x59) 108.0MHz +HSync +VSync
        h: width 1152 start 1216 end 1344 total 1600 skew 0 clock 67.5KHz
        v: height 864 start 865 end 868 total 900 clock 75.0Hz
  1152x864 (0x5a) 104.0MHz -HSync +VSync
        h: width 1152 start 1224 end 1344 total 1536 skew 0 clock 67.7KHz
        v: height 864 start 867 end 871 total 905 clock 74.8Hz
  1024x768 (0x5b) 78.8MHz +HSync +VSync
        h: width 1024 start 1040 end 1136 total 1312 skew 0 clock 60.1KHz
        v: height 768 start 769 end 772 total 800 clock 75.1Hz
  1024x768 (0x5c) 75.0MHz -HSync -VSync
        h: width 1024 start 1048 end 1184 total 1328 skew 0 clock 56.5KHz
        v: height 768 start 771 end 777 total 806 clock 70.1Hz
  1024x768 (0x4f) 65.0MHz -HSync -VSync
        h: width 1024 start 1048 end 1184 total 1344 skew 0 clock 48.4KHz
        v: height 768 start 771 end 777 total 806 clock 60.0Hz
  832x624 (0x5d) 57.3MHz -HSync -VSync
        h: width 832 start 864 end 928 total 1152 skew 0 clock 49.7KHz
        v: height 624 start 625 end 628 total 667 clock 74.6Hz
  800x600 (0x5e) 50.0MHz +HSync +VSync
        h: width 800 start 856 end 976 total 1040 skew 0 clock 48.1KHz
        v: height 600 start 637 end 643 total 666 clock 72.2Hz
  800x600 (0x5f) 49.5MHz +HSync +VSync
        h: width 800 start 816 end 896 total 1056 skew 0 clock 46.9KHz
        v: height 600 start 601 end 604 total 625 clock 75.0Hz
  800x600 (0x50) 40.0MHz +HSync +VSync
        h: width 800 start 840 end 968 total 1056 skew 0 clock 37.9KHz
        v: height 600 start 601 end 605 total 628 clock 60.3Hz
  640x480 (0x60) 31.5MHz -HSync -VSync
        h: width 640 start 656 end 720 total 840 skew 0 clock 37.5KHz
        v: height 480 start 481 end 484 total 500 clock 75.0Hz
  640x480 (0x61) 31.5MHz -HSync -VSync
        h: width 640 start 664 end 704 total 832 skew 0 clock 37.9KHz
        v: height 480 start 489 end 491 total 520 clock 72.8Hz
  640x480 (0x62) 30.2MHz -HSync -VSync
        h: width 640 start 704 end 768 total 864 skew 0 clock 35.0KHz
        v: height 480 start 483 end 486 total 525 clock 66.7Hz
  640x480 (0x63) 25.2MHz -HSync -VSync
        h: width 640 start 656 end 752 total 800 skew 0 clock 31.5KHz
        v: height 480 start 490 end 492 total 525 clock 60.0Hz
  720x400 (0x64) 28.3MHz -HSync +VSync
        h: width 720 start 738 end 846 total 900 skew 0 clock 31.5KHz
        v: height 400 start 412 end 414 total 449 clock 70.1Hz
  native (0x6d) 160.9MHz
        h: width 1920 start 1980 end 2040 total 2160 skew 0 clock 74.5KHz
        v: height 1200 start 1201 end 1204 total 1242 clock 60.0Hz
S-video disconnected (normal left inverted right)
        Identifier: 0x4e
        Timestamp: 2036037239
        Subpixel: no subpixels
        Clones:
        CRTCs: 0 1
                tv_standard: ntsc
        tv_vertical_position: 0 (0x00000000) range: (-5,5)
        tv_horizontal_position: 0 (0x00000000) range: (-5,5)
        tv_horizontal_size: 0 (0x00000000) range: (-5,5)
        load_detection: 0 (0x00000000) range: (0,1)