The wayland protocol lets clients set a surface region to indicate that that region contains all opaque pixels.  The idea is that in the common case, most apps have rounded corners and drop shadows, but in the center is a big opaque rectangle.  If the compositor knows about that region, it can optimize rendering to not paint surfaces below the opaque region and turn off blending.

In the general case, this may be expensive to compute and there's also the discussion of how fine-grained the region should be vs how many opaque pixels it covers.

For the typical window, there will be a big opaque rectangle in the middle that we can communicate to the compositor.