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### 37.14 Scroll Bars

Normally the frame parameter `vertical-scroll-bars` controls whether the windows in the frame have vertical scroll bars, and whether they are on the left or right. The frame parameter `scroll-bar-width` specifies how wide they are (`nil` meaning the default).

The frame parameter `horizontal-scroll-bars` controls whether the windows in the frame have horizontal scroll bars. The frame parameter `scroll-bar-height` specifies how high they are (`nil` meaning the default). See Layout Parameters.

Horizontal scroll bars are not available on all platforms. The function `horizontal-scroll-bars-available-p` which takes no argument returns non-`nil` if they are available on your system.

The following three functions take as argument a live frame which defaults to the selected one.

Function: frame-current-scroll-bars &optional frame

  This function reports the scroll bar types for frame frame. The value is a cons cell `(vertical-type . horizontal-type)`, where vertical-type is either `left`, `right`, or `nil` (which means no vertical scroll bar.) horizontal-type is either `bottom` or `nil` (which means no horizontal scroll bar).

Function: frame-scroll-bar-width &optional Lisp_Object &optional frame

  This function returns the width of vertical scroll bars of frame in pixels.

Function: frame-scroll-bar-height &optional Lisp_Object &optional frame

  This function returns the height of horizontal scroll bars of frame in pixels.

You can override the frame specific settings for individual windows by using the following function:

Function: set-window-scroll-bars window &optional width vertical-type height horizontal-type

  This function sets the width and/or height and the types of scroll bars for window window.

  width specifies the width of the vertical scroll bar in pixels (`nil` means use the width specified for the frame). vertical-type specifies whether to have a vertical scroll bar and, if so, where. The possible values are `left`, `right`, `t`, which means to use the frame’s default, and `nil` for no vertical scroll bar.

  height specifies the height of the horizontal scroll bar in pixels (`nil` means use the height specified for the frame). horizontal-type specifies whether to have a horizontal scroll bar. The possible values are `bottom`, `t`, which means to use the frame’s default, and `nil` for no horizontal scroll bar.

  If window is `nil`, the selected window is used.

The following four functions take as argument a live window which defaults to the selected one.

Function: window-scroll-bars &optional window

  This function returns a list of the form `(width columns vertical-type height lines horizontal-type)`.

  The value width is the value that was specified for the width of the vertical scroll bar (which may be `nil`); columns is the (possibly rounded) number of columns that the vertical scroll bar actually occupies.

  The value height is the value that was specified for the height of the horizontal scroll bar (which may be `nil`); lines is the (possibly rounded) number of lines that the horizontally scroll bar actually occupies.

Function: window-current-scroll-bars &optional window

  This function reports the scroll bar type for window window. The value is a cons cell `(vertical-type . horizontal-type)`. Unlike `window-scroll-bars`, this reports the scroll bar type actually used, once frame defaults and `scroll-bar-mode` are taken into account.

Function: window-scroll-bar-width &optional window

  This function returns the width in pixels of window’s vertical scrollbar.

Function: window-scroll-bar-height &optional window

  This function returns the height in pixels of window’s horizontal scrollbar.

If you don’t specify these values for a window with `set-window-scroll-bars`, the buffer-local variables `vertical-scroll-bar`, `horizontal-scroll-bar`, `scroll-bar-width` and `scroll-bar-height` in the buffer being displayed control the window’s scroll bars. The function `set-window-buffer` examines these variables. If you change them in a buffer that is already visible in a window, you can make the window take note of the new values by calling `set-window-buffer` specifying the same buffer that is already displayed.

You can control the appearance of scroll bars for a particular buffer by setting the following variables which automatically become buffer-local when set.

Variable: vertical-scroll-bar

  This variable specifies the location of the vertical scroll bar. The possible values are `left`, `right`, `t`, which means to use the frame’s default, and `nil` for no scroll bar.

Variable: horizontal-scroll-bar

  This variable specifies the location of the horizontal scroll bar. The possible values are `bottom`, `t`, which means to use the frame’s default, and `nil` for no scroll bar.

Variable: scroll-bar-width

  This variable specifies the width of the buffer’s vertical scroll bars, measured in pixels. A value of `nil` means to use the value specified by the frame.

Variable: scroll-bar-height

  This variable specifies the height of the buffer’s horizontal scroll bar, measured in pixels. A value of `nil` means to use the value specified by the frame.

Finally you can toggle the display of scroll bars on all frames by customizing the variables `scroll-bar-mode` and `horizontal-scroll-bar-mode`.

User Option: scroll-bar-mode

  This variable controls whether and where to put vertical scroll bars in all frames. The possible values are `nil` for no scroll bars, `left` to put scroll bars on the left and `right` to put scroll bars on the right.

User Option: horizontal-scroll-bar-mode

  This variable controls whether to display horizontal scroll bars on all frames.

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