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Unhandled Win32 messages caused a drag-and-drop cleanup leak

devblogs.microsoft.com by Raymond Chen Editor score 7/10

A time-travel trace showed that messages arriving after WM_DESTROY reached a window procedure that expected state already cleaned up; failing to pass unhandled messages to DefWindowProc was the root issue. The case shows how teardown paths can expose bugs in long-established API usage.

What

A Windows memory leak report involving RegisterDragDrop and RevokeDragDrop was traced to a window procedure's message handling.

How

After WM_DESTROY, the procedure returned 0 for later unhandled messages instead of passing them to DefWindowProc, which performs cleanup.

Why it matters

Window procedures must forward unhandled messages; system cleanup can depend on default handling even after application state is gone.

Read on devblogs.microsoft.com →

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