What broke
A capture read the client area of a window through the platform's device context, which returns what is composited inside the window's rectangle — so an overlapping window put its own pixels into the evidence, and a later capture run's occlusion refusal fired legitimately and wrote nothing.
The rule it paid for
Exclusive foreground* is a measurement, not an implementation choice. A window pushed behind another is not a slower measurement; it is a false one.