What "Not Responding" Really Is — How Windows Decides an App Has Hung, and How to Design Apps That Don't (archived 2026-08-30)

Abstract

Windows' "Not Responding" is a mechanism in which the OS judges that a window has not retrieved a message for 5 seconds and replaces it with a ghost window. This article covers the internals of that judgment, the classic causes of hangs, designs that move heavy work off the UI thread, and a procedure for investigating hangs. Archived version of https://comcomponent.com/en/blog/windows-app-not-responding-hang-mechanism/, as published on 2026-08-30. The live article is maintained and may change after this date. First published 2026-08-22.