What makes a process appear as Not responding in Windows?

What is it exactly that "triggers" Windows to mark a process as Not responding in the Task Manager and Resource Monitor?

How does Windows know a program is not responding?

The Answer An application gets events from a queue provided by Windows. If the application does not poll the event queue for a while (5 seconds), such as doing a long calculation for example, then Windows assumes that the application is hung and alerts the user.

What does it mean when an application is not responding?

“Not responding” happens when a program stops interacting with Windows properly. This can be due to legitimate delays, as the program is busy working. It can also be due to errors of various sorts including a programming bug, a design error, a hardware problem, malware, and more.

The API behind this is IsHungAppWindow. Basically, if the application has not pumped a message within 5 seconds it can be marked as not responding.