Comment 87 for bug 263661

Replacing the entire body of PreparePatternForUntiledDrawing with

{
    aPattern->SetExtend(gfxPattern::EXTEND_NONE);
    aPattern->SetFilter(gfxPattern::FILTER_FAST);
}

reproduces the 3.6 behavior. Replacing it with

{
    aPattern->SetExtend(gfxPattern::EXTEND_PAD);
    aPattern->SetFilter(gfxPattern::FILTER_FAST);
}

produces behavior which is _nearly_ correct; the image itself is upscaled correctly (instead of being gradientified and fattened), and there aren't black lines under the image at any zoom level, but some of the lines still get black blobs at their right edges at some zoom levels, and also now sometimes at their left edges. However, this happens much less often.

Based on this I feel relatively confident in diagnosing a video driver bug, and my recommendation is still to rip out the entire mess and use EXTEND_PAD and a good filter unconditionally, then to work with the X.org folks to get the video drivers fixed.