I'm not even sure that region
highlighting is implemented as an overlay; it may be a text property, and,
correct me if I'm wrong, overlays always have higher priority, regardless
of their 'priority property.
Last I looked, region-highlighting was implemented with special C
support, and always overrides all `face' properties. However,
`mouse-face' highlighting is also implemented with special C code.
Although it is controlled by text and overlay properties, the bounds
are recorded, and the highlighting itself is handled specially in the
signal handler so it can respond to low level mouse-motion X events.
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