I have a problem very similar to the one described here: https://microsoft.public.win32.programmer.kernel.narkive.com/Ly2P8Yp2/prevent-vista-from-marking-my-application-as-non-responding
That thread suggests that Task Manager sends WM_NULL to the process and expects the process to consume this message within timeout limit (5 seconds?). When I google for "WM_NULL hung" there are many references to the same technique.
However I don't see any WM_NULL messages in the queue of my application while it works on a lengthy operation - I have a secondary thread that switches to the main thread every 0.5 sec and calls PeekMessage() looking for WM_NULL, and it doesn't find any!
So, what's the method that Windows (Vista) uses to determine if an application is hung?
Which messages should my application consume so that Windows thinks that the application is responsive?
MORE DETAILS :
Along with PeekMessage() looking for WM_NULL, we also call PeekMessage() for mouse events, since we also want to understand if the user picked certain area of the window, where a stop sign is drawn. If the area is picked, we set a flag that the lengthy operation in the main thread periodically checks, and will stop if the stop sign is picked. The problem with Vista is that when it declares application as unresponsive, it replaces its window with a ghost window - see description of PeekMessage() :
If a top-level window stops responding to messages for more than several seconds, the system considers the window to be not responding and replaces it with a ghost window that has the same z-order, location, size, and visual attributes. This allows the user to move it,