On some non-U.S. English keyboards, certain keys are defined to add a diacritic to a letter. These are called “dead keys” because they don’t generate characters by themselves. For instance, when a German keyboard is installed, the key that is in the same position as the +/= key on a U.S. keyboard is a dead key for the grave accent (`) when shifted and the acute accent (´) when unshifted.

When a user presses this dead key, your window procedure receives a WM_DEADCHAR message with wParam equal to ASCII or Unicode code for the diacritic by itself. When the user then presses a letter key that can be written with this diacritic (for instance, the A key), the window procedure receives a WM_CHAR message where wParam is the ANSI code for the letter `a’ with the diacritic.

Thus, your program does not have to process the WM_DEADCHAR message because the WM_CHAR message gives the program all the information it needs. The Windows logic even has built-in error handling: If the dead key is followed by a letter that can’t take a diacritic (such as `s’), the window procedure receives two WM_CHAR messages in a row—the first with wParam equal to the ASCII code for the diacritic by itself (the same wParam value delivered with the WM_DEADCHAR message) and the second with wParam equal to the ASCII code for the letter `s’.

Of course, the best way to get a feel for this is to see it in action. You need to load a foreign keyboard that uses dead keys, such as the German keyboard that I described earlier. You do this in the Control Panel by selecting Keyboard and then the Language tab. Then you need an application that shows you the details of every keyboard message a program can receive.

WM_VSCROLL and WM_HSCROLL are accompanied by the wParam and lParam message parameters. For messages from scroll bars created as part of your window, you can ignore lParam; that’s used only for scroll bars created as child windows, usually within dialog boxes.

The wParam message parameter is divided into a low word and a high word. The low word of wParam is a number that indicates what the mouse is doing to the scroll bar. This number is referred to as a “notification code.” Notification codes have values defined by identifiers that begin with SB, which stands for “scroll bar.” Here’s how the notification codes are defined in WINUSER.H:

#define SB_LINEUP           0
#define SB_LINELEFT         0
#define SB_LINEDOWN         1
#define SB_LINERIGHT        1
#define SB_PAGEUP           2
#define SB_PAGELEFT         2
#define SB_PAGEDOWN         3
#define SB_PAGERIGHT        3
#define SB_THUMBPOSITION    4
#define SB_THUMBTRACK       5
#define SB_TOP              6
#define SB_LEFT             6
#define SB_BOTTOM           7
#define SB_RIGHT            7
#define SB_ENDSCROLL        8

If you hold down the mouse button on the various parts of the scroll bar, your program can receive multiple scroll bar messages. When the mouse button is released, you’ll get a message with a notification code of SB_ENDSCROLL. You can generally ignore messages with the SB_ENDSCROLL notification code. Windows will not change the position of the scroll bar thumb. Your application does that by calling SetScrollPos.

When you position the mouse cursor over the scroll bar thumb and press the mouse button, you can move the thumb. This generates scroll bar messages with notification codes of SB_THUMBTRACK and SB_THUMBPOSITION. When the low word of wParam is SB_THUMBTRACK, the high word of wParam is the current position of the scroll bar thumb as the user is dragging it. This position is within the minimum and maximum values of the scroll bar range. When the low word of wParam is SB_THUMBPOSITION, the high word of wParam is the final position of the scroll bar thumb when the user released the mouse button. For other scroll bar actions, the high word of wParam should be ignored.

To provide feedback to the user, Windows will move the scroll bar thumb when you drag it with the mouse as your program is receiving SB_THUMBTRACK messages. However, unless you process SB_THUMBTRACK or SB_THUMBPOSITION messages by calling SetScrollPos, the thumb will snap back to its original position when the user releases the mouse button.

A program can process either the SB_THUMBTRACK or SB_THUMBPOSITION messages, but doesn’t usually process both. If you process SB_THUMBTRACK messages, you’ll move the contents of your client area as the user is dragging the thumb. If instead you process SB_THUMBPOSITION messages, you’ll move the contents of the client area only when the user stops dragging the thumb.

The WINUSER.H header files includes notification codes of SB_TOP, SB_BOTTOM, SB_LEFT, and SB_RIGHT, indicating that the scroll bar has been moved to its minimum or maximum position. However, you will never receive these notification codes for a scroll bar created as part of your application window.

Although it’s not common, using 32-bit values for the scroll bar range is perfectly valid. However, the high word of wParam, which is only a 16-bit value, cannot properly indicate the position for SB_THUMBTRACK and SB_THUMBPOSITION actions. In this case, you need to use the function GetScrollInfo