Created attachment 4586 [details]
Sample document to reproduce the bug

Sorry about the phenomenal bug spam, guys.  Adding xorg-team@ to the QA contact so bugs don't get lost in future.

Matthieu Herrb
Do you still experience this issue with newer soft ?
Please check the status of your issue.

From the description in the example, and a quick look at the magicpoint code, I think this is a bug in magicpoint itself:

        vinfo = XGetVisualInfo(display, vinfo_mask, &vinfo_template, &ninfo);
        best = NULL;
        for (i = 0; i < ninfo; i++) {
                switch (vinfo[i].class) {
                case TrueColor:
                        if (vinfo[i].depth < 15 || 24 < vinfo[i].depth)
                                break;
                        if (best == NULL ||
                            best->class != TrueColor ||
                            best->depth < vinfo[i].depth)
                                best = &vinfo[i];
                        break;
                case PseudoColor:
                        if (best == NULL)
                                best = &vinfo[i];
                        break;
                }
        }

That's going to pick the ARGB visual, and (0,0,0,0) is transparent black.  Either run magicpoint with XLIB_SKIP_ARGB_VISUALS=1, or fix that loop.