Wide LineDoubleDash lines drew black on screen under XQuartz while the server's own XGetImage readback showed the
correct pixels, because at depth 24 that readback ignores alpha. Opaque text and dashed arcs take the same path.
fbValidateGC widens a planemask that covers the drawable's depth to cover its full bits per pixel, so the blit and
fill paths can take their all-ones shortcuts. On a rootless server that also writes the alpha channel of the window's
premultiplied surface, which CoreGraphics then composites as black. Rootless keeps that alpha opaque by clearing the
bits from the planemask, but only its own ValidateGC does so, and mi revalidates the GC in the middle of a drawing
operation to swap the foreground for the background pixel. By then the damage layer has unwrapped the rootless funcs
down to fb, so the reassertion never happens and the rest of the operation lands with a zero alpha channel.
Fixes: https://github.com/XQuartz/XQuartz/issues/230
Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
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