The people who work on the next generation of the window manager, known as the Desktop Window Manager (DWM), told me that their original plan was to get rid of the compatibility hack that says that invalidating the null window invalidates the entire desktop, but by an amazing coincidence, two days after I posted that article, they received a report that the beta version of an upcoming product from a major vendor still relies on that behavior (albeit accidentally). They contacted the vendor who agreed to fix the bug, but the fact that a modern program still relies on this ancient behavior gave them pause. If a program written just this year relies on the null window hack, imagine how many programs written in years past also rely on that behavior. After some deliberation, they decided to put the compatibility hack back in, just to be safe.
Some compatibility hacks never die, no matter how hard you try to kill them.