Win32: My Application freezes while the user resizes the window

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There are a number of modal operations that happen on windows. Win32 Modal operations refer to functions that put an application into a “mode” by starting their own event processing loop until the mode finishes. Common application modes include drag and drop operations, move/size operations, anytime a dialog pops up that needs input before the application can continue.

So what is happening is: Your message loop is NOT being run.
Your window recieved a WM_LBUTTONDOWN message that you passed to DefWindowProc. DefWindowProc determined that the user was trying to size or move the window interactively and entered a sizing/moving modal function. This function is in a message processing loop watching for mouse messages so that It can intercept them to provide the interactive sizing experience, and will only exit when the sizing operation completes – typically by the user releasing the held button, or by pressing escape.

You get notified of this – DefWindowProc sends a WM_ENTERSIZEMOVE and WM_EXITSIZEMOVE messages as it enters and exits the modal event processing loop.

To continue to generate “idle” messages, typically create a timer (SetTimer) before calling a modal function – or when getting a message that DefWindowProc is entering a modal function – the modal loop will continue to dispatch WM_TIMER messages… and call the idle proc from the timer message handler. Destroy the timer when the modal function returns.

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