What is checked before a file exists
- ✓The z order above the window is enumerated and each frame intersected with the copy rectangle — one pass that answers for the whole area, where sampled points cannot.
- ✓An overlap fails rather than crops, and the refusal names the intruder, its process and the rectangle it covers.
- ✓A window that opted into a system backdrop is refused rather than warned about: it transmits what is behind it, and z-order reasoning cannot answer for glass.
- ✓A copy of exactly one colour is not a picture of a window — that is what a session where nothing was rendering produced, and it exited zero.
- ✓A page still computing is not a report: the loading strings are read from the project's own language files before anything launches.
- ✓A capture of a window this run is not driving is refused, because a picture of somebody else's desktop is not evidence about your application.
Two renders of unchanged code are byte-identical
Which is what makes a difference a real difference, and means nobody has to choose a tolerance for a comparison to mean anything. A change meant to be invisible finally has a cheap way to prove it was.
Measured, arranged, updated — in that order
The arrange is why the render verb exists. A tree that was measured and never arranged renders as a fully transparent picture of exactly the right size, which looks like a drawing bug and is a calling bug.