Vertical fractional scrolling means shifting text in a window up or down by a specified multiple or fraction of a line. Emacs uses it, for example, on images and screen lines which are taller than the window. Each window has a vertical scroll position, which is a number, never less than zero. It specifies how far to raise the contents of the window when displaying them. Raising the window contents generally makes all or part of some lines disappear off the top, and all or part of some other lines appear at the bottom. The usual value is zero.
The vertical scroll position is measured in units of the normal line height, which is the height of the default font. Thus, if the value is .5, that means the window contents will be scrolled up half the normal line height. If it is 3.3, that means the window contents are scrolled up somewhat over three times the normal line height.
What fraction of a line the vertical scrolling covers, or how many lines, depends on what the lines contain. A value of .5 could scroll a line whose height is very short off the screen, while a value of 3.3 could scroll just part of the way through a tall line or an image.
This function returns the current vertical scroll position of
window. The default for window is the selected window.
If pixels-p is non-nil, the return value is measured in
pixels, rather than in units of the normal line height.
(window-vscroll)
⇒ 0
This function sets window’s vertical scroll position to
lines. If window is nil, the selected window is
used. The argument lines should be zero or positive; if not, it
is taken as zero.
The actual vertical scroll position must always correspond to an integral number of pixels, so the value you specify is rounded accordingly.
The return value is the result of this rounding.
(set-window-vscroll (selected-window) 1.2)
⇒ 1.13
If pixels-p is non-nil, lines specifies a number of
pixels. In this case, the return value is lines.
Normally, the vscroll does not take effect on windows that aren’t the
minibuffer-scroll-window or the selected window when the
mini-window is resized (see Minibuffer Windows). This “frozen”
behavior is disabled when the preserve-vscroll-p parameter is
non-nil, which means to set the vscroll as usual.
If this variable is non-nil, the line-move,
scroll-up, and scroll-down functions will automatically
modify the vertical scroll position to scroll through display rows
that are taller than the height of the window, for example in the
presence of large images.