I'm not sure whether that's a good idea to use QMargins for measures, maybe, it would be better to have something like

struct {
    QSize left;
    QSize top;
    QSize right;
    QSize bottom;
} sizes, fixedSizes;

in FrameData, which then should go in a different diff (if you think it's worth it).

Also, I think it will simplify generateFrameBackground a little bit, e.g.

paintBorder(p, frame, FrameSvg::RightBorder, frame->sizes.right * q->devicePixelRatio, contentRect);

so there won't be a need for const int leftHeight = q->elementSize(frame->prefix % QLatin1String("left")).height();