The keyboard focus is a temporary attribute of a window; the window that has the keyboard focus receives all keyboard input until the focus changes to a different window. The system converts keyboard input events into WM_CHAR messages and posts them to the message queue of the window that has the keyboard focus.

An application can set the keyboard focus to a particular window by calling WinSetFocus. If the application does not use WinSetFocus to explicitly set the keyboard-focus window, the system sets the focus to the active frame window.

The following events occur when an application uses WinSetFocus to shift the keyboard focus from one window (the original window) to another (the new window):

• The system sends the original window a WM_SETFOCUS message (with the fFocus parameter set to FALSE), indicating that that window has lost the keyboard focus.

• The system then sends the original window a WM_SETSELECTION message, indicating that the window should remove the highlight from the current selection.

• If the original (frame) window is being deactivated, the system sends it a WM_ACTIVATE message (with the fActive parameter set to FALSE), indicating that the window is no longer active.

• The system then sends the new application a WM_ACTIVATE message (with fActive set to TRUE), indicating that the new application is now active.

• If the new (main) window is being activated, the system sends it a WM_ACTIVATE message (with fActive set to TRUE), indicating that the main window is now active.

• The system sends the new window a WM_SETSELECTION message, indicating that the window should highlight the current selection.

• Finally, the system sends the new window a WM_SETFOCUS message (with fFocus set to TRUE), indicating that the new window has the keyboard focus.

If, while processing a WM_SETFOCUS message, an application calls WinQueryActiveWindow, that function returns the handle of the previously-active window until the application establishes a new active window. Similarly, if the application, while processing WM_SETFOCUS, calls WinQueryFocus, that function returns the handle of the previous keyboard-focus window until the application establishes a new keyboard-focus window. In other words, even though the system has sent WM_ACTIVATE and WM_SETFOCUS messages (with the fActive and fFocus parameters set to FALSE) to the previous windows, those windows are considered the active and focus windows until the system establishes new active and focus windows.

If the application calls WinSetFocus while processing a WM_ACTIVATE message, the system does not send a WM_SETFOCUS message (with fFocus set to FALSE), because no window has the focus.

A client window receives a WM_ACTIVATE message when its parent frame window is being activated or deactivated. The activation or deactivation message usually is followed by a WM_SETFOCUS message that specifies whether the client window is gaining or losing the keyboard focus. Therefore, if the client window needs to change the keyboard focus, it should do so during the WM_SETFOCUS message, not during the WM_ACTIVATE message.