Win32 WM_PAINT and a child window

Answer 1

Do you return 0 after doing the WM_PAINT handling? Beyond that ... show us your wndprocs and your Create and your RegisterClass calls.

Answer 2

You should be calling BeginPaint and EndPaint in response to the WM_PAINT message to validate the window. Otherwise, the system thinks that your window has not been painted and so it will send the paint message again (and again). See the Microsoft documentation.