.section Scrolling

Scrolling is a general facility a window may have for vertically
positioning what it displays, so that a larger display may be viewed
than will fit in the window.  Scrolling is controlled by the mouse in
various ways, depending on just which mixins are included.

.defmessage :scroll-absolute to
Move the line or item numbered 2to* to the top of the window, by sending a 3:scroll-to*
message.
.end_defmessage

.defmessage :scroll-relative from to
2from* and 2to* are one of the following:
.table 3
.item :top
The top of the window.
.item :bottom
The bottom of the window.
.item 1A fixnum*
A number of pixels from the top of the window (2outside* the top margin).
.end_table
Moves the line or item specified by 2from* to the position specified by
2to*, by sending a 3:scroll-to* message.
.end_defmessage

.defmessage :scroll-to to &optional (type ':absolute)
2type* is one of:
.table 3
.item :absolute
Places the line or item numbered 2to* at the top of the window.
.item :relative
Adjusts the line or item displayed at the top of the window 2to*.
.end_table
Since 2to* is not guaranteed to be legal, both types of scrolling must
error check their arguments.  In general, all scrolling is implemented via
this method, and 3:scroll-absolute* and 3:scroll-relative* merely
massage their arguments and send this message.
.end_defmessage

.defmessage :scroll-position
Returns four values: 
.table 3
.item top-line-num
The line-number (or item number) of the line (item) currently at the top of the window.
.item total-lines
The total number of lines (items) available to scroll through.
.item line-height
The height (in pixels) of a line (item).
.item n-items
The number of items current displayed on the window.
.end_table
.end_defmessage

.defmessage :new-scroll-position
Sent whenever the item at the top of the window changes, and scrolling is "enabled"
(the scroll bar is displayed, for example).  Responsible for updating any information
that depends upon the scroll position.  Typically this redisplays the scroll bar.
.end_defmessage

.defmessage :scroll-bar-p
Returns 3t* if the window supports a scroll bar.
.end_defmessage

.defmessage :scroll-bar
.defmessage1 :set-scroll-bar new-scroll-bar
Get or set the value of the instance variable 3scroll-bar*, which is a description
of how and where the scroll bar is to be displayed.
[Why is this available to the user?  Should it be documented?  If so,
the format of the value of the instance variable should be documented.]
.end_defmessage

.defmessage :scroll-bar-always-displayed
.defmessage1 :set-scroll-bar-always-displayed t-or-nil
Get or set the value of the instance variable 3scroll-bar-always-displayed*.
If the variable is 3t*, then the scroll bar is displayed even when the
mouse is not within the
scroll bar's region.  If 3nil*, the scroll bar is only displayed when the mouse
moves into its active area.
.end_defmessage