About Mouse Input

The mouse is an important, but optional, user-input device for applications. A well-written application should include a mouse interface, but it should not depend solely on the mouse for acquiring user input. The application should provide full keyboard support as well.

An application receives mouse input in the form of messages that are sent or posted to its windows.

This section covers the following topics:

Mouse Cursor

When the user moves the mouse, the system moves a bitmap on the screen called the mouse cursor. The mouse cursor contains a single-pixel point called the hot spot, a point that the system tracks and recognizes as the position of the cursor. When a mouse event occurs, the window that contains the hot spot typically receives the mouse message resulting from the event. The window need not be active or have the keyboard focus to receive a mouse message.

The system maintains a variable that controls mouse speed—that is, the distance the cursor moves when the user moves the mouse. You can use the SystemParametersInfo function with the SPI_GETMOUSE or SPI_SETMOUSE flag to retrieve or set mouse speed. For more information about mouse cursors, see Cursors.

Mouse Capture

The system typically posts a mouse message to the window that contains the cursor hot spot when a mouse event occurs. An application can change this behavior by using the SetCapture function to route mouse messages to a specific window. The window receives all mouse messages until the application calls the ReleaseCapture function or specifies another capture window, or until the user clicks a window created by another thread.

When the mouse capture changes, the system sends a WM_CAPTURECHANGED message to the window that is losing the mouse capture. The lParam parameter of the message specifies a handle to the window that is gaining the mouse capture.

Only the foreground window can capture mouse input. When a background window attempts to capture mouse input, it receives messages only for mouse events that occur when the cursor hot spot is within the visible portion of the window.

Capturing mouse input is useful if a window must receive all mouse input, even when the cursor moves outside the window. For example, an application typically tracks the cursor position after a mouse button down event, following the cursor until a mouse button up event occurs. If an application has not captured mouse input and the user releases the mouse button outside the window, the window does not receive the button-up message.

A thread can use the GetCapture function to determine whether one of its windows has captured the mouse. If one of the thread's windows has captured the mouse, GetCapture retrieves a handle to the window.

Mouse ClickLock

The Mouse ClickLock accessibility feature enables a user lock down the primary mouse button after a single click. To an application, the button still appears to be pressed down. To unlock the button, an application can send any mouse message or the user can click any mouse button. This feature lets a user do complex mouse combinations more simply. For example, those with certain physical limitations can highlight text, drag objects, or open menus more easily. For more information, see the following flags and the Remarks in SystemParametersInfo:


Mouse Configuration

Although the mouse is an important input device for applications, not every user necessarily has a mouse. An application can determine whether the system includes a mouse by passing the SM_MOUSEPRESENT value to the GetSystemMetrics function.

Windows supports a mouse having up to three buttons. On a three-button mouse, the buttons are designated as the left, middle, and right buttons. Messages and named constants related to the mouse buttons use the letters L, M, and R to identify the buttons. The button on a single-button mouse is considered to be the left button. Although Windows supports a mouse with multiple buttons, most applications use the left button primarily and the others minimally, if at all.

Applications can also support a mouse wheel. The mouse wheel can be pressed or rotated. When the mouse wheel is pressed, it acts as the middle (third) button, sending normal middle button messages to your application. When it is rotated, a wheel message is sent to your application. For more information, see The Mouse Wheel section.

Applications can support application-command buttons. These buttons, called X buttons, are designed to allow easier access to an Internet browser, electronic mail, and media services. When an X button is pressed, a WM_APPCOMMAND message is sent to your application. For more information, see the description in the WM_APPCOMMAND message.

An application can determine the number of buttons on the mouse by passing the SM_CMOUSEBUTTONS value to the GetSystemMetrics function. To configure the mouse for a left-handed user, the application can use the SwapMouseButton function to reverse the meaning of the left and right mouse buttons. Passing the SPI_SETMOUSEBUTTONSWAP value to the SystemParametersInfo function is another way to reverse the meaning of the buttons. Note, however, that the mouse is a shared resource, so reversing the meaning of the buttons affects all applications.


Windows supports a mouse with five buttons. In addition to the left, middle, and right buttons there are XBUTTON1 and XBUTTON2, which provide backward and forward navigation when using your browser.

The window manager supports XBUTTON1 and XBUTTON2 through the WM_XBUTTON* and WM_NCXBUTTON* messages. The HIWORD of the WPARAM in these messages contains a flag indicating which X button was pressed. Because these mouse messages also fit between the constants WM_MOUSEFIRST and WM_MOUSELAST, an application can filter all mouse messages with GetMessage or PeekMessage.

The following support XBUTTON1 and XBUTTON2:

The following APIs were modified to support these buttons:

It is unlikely that a child window in a component application will be able to directly implement commands for the XBUTTON1 and XBUTTON2. So DefWindowProc sends a WM_APPCOMMAND message to a window when an X button is clicked. DefWindowProc also sends the WM_APPCOMMAND message to its parent window. This is similar to the way context menus are invoked with a right click—DefWindowProc sends a WM_CONTEXTMENU message to the menu and also sends it to its parent. Additionally, if DefWindowProc receives a WM_APPCOMMAND message for a top-level window, it calls a shell hook with code HSHELL_APPCOMMAND.

There is support for the keyboards that have extra keys for browser functions, media functions, application launching, and power management. For more information, see Keyboard Keys for Browsing and Other Functions.

Mouse Messages

The mouse generates an input event when the user moves the mouse, or presses or releases a mouse button. The system converts mouse input events into messages and posts them to the appropriate thread's message queue. When mouse messages are posted faster than a thread can process them, the system discards all but the most recent mouse message.

A window receives a mouse message when a mouse event occurs while the cursor is within the borders of the window, or when the window has captured the mouse. Mouse messages are divided into two groups: client area messages and nonclient area messages. Typically, an application processes client area messages and ignores nonclient area messages.

This section covers the following topics:

Client Area Mouse Messages

A window receives a client area mouse message when a mouse event occurs within the window's client area. The system posts the WM_MOUSEMOVE message to the window when the user moves the cursor within the client area. It posts one of the following messages when the user presses or releases a mouse button while the cursor is within the client area.

WM_LBUTTONDBLCLK The left mouse button was double-clicked.
WM_LBUTTONDOWN The left mouse button was pressed.
WM_LBUTTONUP The left mouse button was released.
WM_MBUTTONDBLCLK The middle mouse button was double-clicked.
WM_MBUTTONDOWN The middle mouse button was pressed.
WM_MBUTTONUP The middle mouse button was released.
WM_RBUTTONDBLCLK The right mouse button was double-clicked.
WM_RBUTTONDOWN The right mouse button was pressed.
WM_RBUTTONUP The right mouse button was released.
WM_XBUTTONDBLCLK An X mouse button was double-clicked.
WM_XBUTTONDOWN An X mouse button was pressed.
WM_XBUTTONUP An X mouse button was released.


In addition, an application can call the TrackMouseEvent function to have the system send two other messages. It posts the WM_MOUSEHOVER message when the cursor hovers over the client area for a certain time period. It posts the WM_MOUSELEAVE message when the cursor leaves the client area.

Message Parameters

The lParam parameter of a client area mouse message indicates the position of the cursor hot spot. The low-order word indicates the x-coordinate of the hot spot, and the high-order word indicates the y-coordinate. The coordinates are specified in client coordinates. In the client coordinate system, all points on the screen are specified relative to the coordinates (0,0) of the upper-left corner of the client area.

The wParam parameter contains flags that indicate the status of the other mouse buttons and the CTRL and SHIFT keys at the time of the mouse event. You can check for these flags when mouse-message processing depends on the state of another mouse button or of the CTRL or SHIFT key. The wParam parameter can be a combination of the following values.

MK_CONTROLThe CTRL key is down.
MK_LBUTTONThe left mouse button is down.
MK_MBUTTONThe middle mouse button is down.
MK_RBUTTONThe right mouse button is down.
MK_SHIFTThe SHIFT key is down.
MK_XBUTTON1The first X button is down.
MK_XBUTTON2The second X button is down.


Double-Click Messages

The system generates a double-click message when the user clicks a mouse button twice in quick succession. When the user clicks a button, the system establishes a rectangle centered around the cursor hot spot. It also marks the time at which the click occurred. When the user clicks the same button a second time, the system determines whether the hot spot is still within the rectangle and calculates the time elapsed since the first click. If the hot spot is still within the rectangle and the elapsed time does not exceed the double-click time-out value, the system generates a double-click message.

An application can get and set double-click time-out values by using the GetDoubleClickTime and SetDoubleClickTime functions, respectively. Alternatively, the application can set the double-click–time-out value by using the SPI_SETDOUBLECLICKTIME flag with the SystemParametersInfo function. It can also set the size of the rectangle that the system uses to detect double-clicks by passing the SPI_SETDOUBLECLKWIDTH and SPI_SETDOUBLECLKHEIGHT flags to SystemParametersInfo. Note, however, that setting the double-click–time-out value and rectangle affects all applications.

An application-defined window does not, by default, receive double-click messages. Because of the system overhead involved in generating double-click messages, these messages are generated only for windows belonging to classes that have the CS_DBLCLKS class style. Your application must set this style when registering the window class. For more information, see Window Classes.

A double-click message is always the third message in a four-message series. The first two messages are the button-down and button-up messages generated by the first click. The second click generates the double-click message followed by another button-up message. For example, double-clicking the left mouse button generates the following message sequence:


Because a window always receives a button-down message before receiving a double-click message, an application typically uses a double-click message to extend a task it began during a button-down message. For example, when the user clicks a color in the color palette of Microsoft Paint, Paint displays the selected color next to the palette. When the user double-clicks a color, Paint displays the color and opens the Edit Colors dialog box.

Nonclient Area Mouse Messages

A window receives a nonclient area mouse message when a mouse event occurs in any part of a window except the client area. A window's nonclient area consists of its border, menu bar, title bar, scroll bar, window menu, minimize button, and maximize button.

The system generates nonclient area messages primarily for its own use. For example, the system uses nonclient area messages to change the cursor to a two-headed arrow when the cursor hot spot moves into a window's border. A window must pass nonclient area mouse messages to the DefWindowProc function to take advantage of the built-in mouse interface.

There is a corresponding nonclient area mouse message for each client area mouse message. The names of these messages are similar except that the named constants for the nonclient area messages include the letters NC. For example, moving the cursor in the nonclient area generates a WM_NCMOUSEMOVE message, and pressing the left mouse button while the cursor is in the nonclient area generates a WM_NCLBUTTONDOWN message.

The lParam parameter of a nonclient area mouse message is a structure that contains the x- and y-coordinates of the cursor hot spot. Unlike coordinates of client area mouse messages, the coordinates are specified in screen coordinates rather than client coordinates. In the screen coordinate system, all points on the screen are relative to the coordinates (0,0) of the upper-left corner of the screen.

The wParam parameter contains a hit-test value, a value that indicates where in the nonclient area the mouse event occurred. The following section explains the purpose of hit-test values.


Whenever a mouse event occurs, the system sends a WM_NCHITTEST message to either the window that contains the cursor hot spot or the window that has captured the mouse. The system uses this message to determine whether to send a client area or nonclient area mouse message. An application that must receive mouse movement and mouse button messages must pass the WM_NCHITTEST message to the DefWindowProc function.

The lParam parameter of the WM_NCHITTEST message contains the screen coordinates of the cursor hot spot. The DefWindowProc function examines the coordinates and returns a hit-test value that indicates the location of the hot spot. The hit-test value can be one of the following values.

ValueLocation of hot spot
HTBORDERIn the border of a window that does not have a sizing border.
HTBOTTOMIn the lower-horizontal border of a window.
HTBOTTOMLEFTIn the lower-left corner of a window border.
HTBOTTOMRIGHTIn the lower-right corner of a window border.
HTCAPTIONIn a title bar.
HTCLIENTIn a client area.
HTCLOSEIn a Close button.
HTERROROn the screen background or on a dividing line between windows (same as HTNOWHERE, except that the DefWindowProc function produces a system beep to indicate an error).
HTGROWBOXIn a size box (same as HTSIZE).
HTHELPIn a Help button.
HTHSCROLLIn a horizontal scroll bar.
HTLEFTIn the left border of a window.
HTMENUIn a menu.
HTMAXBUTTONIn a Maximize button.
HTMINBUTTONIn a Minimize button.
HTNOWHEREOn the screen background or on a dividing line between windows.
HTREDUCEIn a Minimize button.
HTRIGHTIn the right border of a window.
HTSIZEIn a size box (same as HTGROWBOX).
HTSYSMENUIn a System menu or in a Close button in a child window.
HTTOPIn the upper-horizontal border of a window.
HTTOPLEFTIn the upper-left corner of a window border.
HTTOPRIGHTIn the upper-right corner of a window border.
HTTRANSPARENTIn a window currently covered by another window in the same thread.
HTVSCROLLIn the vertical scroll bar.
HTZOOMIn a Maximize button.


If the cursor is in the client area of a window, DefWindowProc returns the HTCLIENT hit-test value to the window procedure. When the window procedure returns this code to the system, the system converts the screen coordinates of the cursor hot spot to client coordinates, and then posts the appropriate client area mouse message.

The DefWindowProc function returns one of the other hit-test values when the cursor hot spot is in a window's nonclient area. When the window procedure returns one of these hit-test values, the system posts a nonclient area mouse message, placing the hit-test value in the message's wParam parameter and the cursor coordinates in the lParam parameter.

Mouse Sonar

The Mouse Sonar accessibility feature briefly shows several concentric circles around the pointer when the user presses and releases the CTRL key. This feature helps a user locate the mouse pointer on a screen that is cluttered or with resolution set to high, on a poor quality monitor, or for users with impaired vision. For more information, see the following flags in SystemParametersInfo:



Mouse Vanish

The Mouse Vanish accessibility feature hides the pointer when the user is typing. The mouse pointer reappears when the user moves the mouse. This feature keeps the pointer from obscuring the text being typed, for example, in an e-mail or other document. For more information, see the following flags in SystemParametersInfo:



The Mouse Wheel

The mouse wheel combines the features of a wheel and a mouse button. The wheel has discrete, evenly-spaced notches. When you rotate the wheel, a wheel message is sent to your application as each notch is encountered. The wheel button can also operate as a normal Windows middle (third) button. Pressing and releasing the mouse wheel sends standard WM_MBUTTONUP and WM_MBUTTONDOWN messages. Double clicking the third button sends the standard WM_MBUTTONDBLCLK message.

The mouse wheel is supported through the WM_MOUSEWHEEL message.

Rotating the mouse sends the WM_MOUSEWHEEL message to the focus window. The DefWindowProc function propagates the message to the window's parent. There should be no internal forwarding of the message, since DefWindowProc propagates it up the parent chain until a window that processes it is found.

Determining the Number of Scroll Lines

Applications should use the SystemParametersInfo function to retrieve the number of lines a document scrolls for each scroll operation (wheel notch). To retrieve the number of lines, an application makes the following call:

SystemParametersInfo(SPI_GETWHEELSCROLLLINES, 0, pulScrollLines, 0)

The variable "pulScrollLines" points to an unsigned integer value that receives the suggested number of lines to scroll when the mouse wheel is rotated without modifier keys:

  • If this number is 0, no scrolling should occur.
  • If this number is WHEEL_PAGESCROLL, a wheel roll should be interpreted as clicking once in the page down or page up regions of the scroll bar.
  • If the number of lines to scroll is greater than the number of lines viewable, the scroll operation should also be interpreted as a page down or page up operation.

The default value for the number of scroll lines will be 3. If a user changes the number of scroll lines, by using the Mouse Properties sheet in Control Panel, the operating system broadcasts a WM_SETTINGCHANGE message to all top-level windows with SPI_SETWHEELSCROLLLINES specified. When an application receives the WM_SETTINGCHANGE message, it can then get the new number of scroll lines by calling:

SystemParametersInfo(SPI_GETWHEELSCROLLLINES, 0, pulScrollLines, 0)

Controls that Scroll

The table below lists the controls with scrolling functionality (including scroll lines set by the user).

Edit ControlVertical and horizontal.
List box ControlVertical and horizontal.
Combo boxWhen not dropped down, each scroll retrieves the next or previous item. When dropped down, each scroll forwards the message to the list box, which scrolls accordingly.
CMD (Command line)Vertical.
Tree ViewVertical and horizontal.
List ViewVertical and horizontal.
Up/down ScrollsOne item at a time.
Trackbar ScrollsOne item at a time.
Microsoft Rich Edit 1.0Vertical. Note, the Exchange client has its own versions of the list view and tree view controls that do not have wheel support.
Microsoft Rich Edit 2.0Vertical.


Detecting a Mouse with a Wheel

To determine if a mouse with a wheel is connected, call GetSystemMetrics with SM_MOUSEWHEELPRESENT. A return value of TRUE indicates that the mouse is connected.

The following example is from the window procedure for a multiline edit control:

BOOL ScrollLines(
     PWNDDATA pwndData,   //scrolls the window indicated
     int cLinesToScroll); //number of times

short gcWheelDelta; //wheel delta from roll
PWNDDATA pWndData; //pointer to structure containing info about the window
UINT gucWheelScrollLines=0;//number of lines to scroll on a wheel rotation

gucWheelScrollLines = SystemParametersInfo(SPI_GETWHEELSCROLLLINES, 

     * Do not handle zoom and datazoom.
    if (wParam & (MK_SHIFT | MK_CONTROL)) {
        goto PassToDefaultWindowProc;

    gcWheelDelta -= (short) HIWORD(wParam);
    if (abs(gcWheelDelta) >= WHEEL_DELTA && gucWheelScrollLines > 0) 
        int cLineScroll;

         * Limit a roll of one (1) WHEEL_DELTA to
         * scroll one (1) page.
        cLineScroll = (int) min(
                (UINT) pWndData->ichLinesOnScreen - 1,

        if (cLineScroll == 0) {

        cLineScroll *= (gcWheelDelta / WHEEL_DELTA);
        assert(cLineScroll != 0);

        gcWheelDelta = gcWheelDelta % WHEEL_DELTA;
        return ScrollLines(pWndData, cLineScroll);


Window Activation

When the user clicks an inactive top-level window or the child window of an inactive top-level window, the system sends the WM_MOUSEACTIVATE message (among others) to the top-level or child window. The system sends this message after posting the WM_NCHITTEST message to the window, but before posting the button-down message. When WM_MOUSEACTIVATE is passed to the DefWindowProc function, the system activates the top-level window and then posts the button-down message to the top-level or child window.

By processing WM_MOUSEACTIVATE, a window can control whether the top-level window becomes the active window as a result of a mouse click, and whether the window that was clicked receives the subsequent button-down message. It does so by returning one of the following values after processing WM_MOUSEACTIVATE.

MA_ACTIVATEActivates the window and does not discard the mouse message.
MA_NOACTIVATEDoes not activate the window and does not discard the mouse message.
MA_ACTIVATEANDEATActivates the window and discards the mouse message.
MA_NOACTIVATEANDEATDoes not activate the window but discards the mouse message.