An application window's response to a mouse click depends on whether the window is active. The first click in an inactive window should activate the window. Subsequent clicks in the active window produce an application-specific action.

A common problem for an application that processes WM_BUTTON1DOWN or similar messages is failing to activate the window or set the keyboard focus. If the window processes WM_CHAR messages, the window procedure should call WinSetFocus to make sure the window receives the keyboard focus and is activated. If the window does not process WM_CHAR messages, the application should call WinSetActiveWindow to activate the window.