The fix is to support interleaving the drawing of zones and frames.
The draw order is determined by the width of the zones/frames.
Larger areas are drawn before smaller ones. This makes sure that
everything is as visible as possible.
The zone border outlines are still drawn on top of all frames.
Solving that is technically more challenging, because we don't want
to use transparency for zone backgrounds because that results in
many possible mixed colors which we want to avoid. Fortunately,
drawing the outlines on top seems to be quite useful anyway.
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