Correct way to implement `pixel-point-and-height-at-unseen-line'?

[Po Lu]

I don't know how to make `pixel-point-and-height-at-unseen-line' work
correctly: if I use `line-pixel-height', it will only report the height
the first individual line inside a display string if there is one above
window start, but if I use `window-text-pixel-size', I have to determine
the region to measure, which is quite troublesome.

For example, window start is often not the beginning of a visual line in
Info buffers, so in order to measure the height of the previous line (or
object), I have to find the beginning of the visual line at window
start, and pass that as the `to' argument to window-text-pixel-size
instead.

This works okay until there is a `line-prefix' property at window start:
in this case, window start is the beginning of a visual line, but it
does not display at the beginning of the line, which means
`window-text-pixel-size' will add the height of the current line to that
of the previous line.  I will add a workaround to pixel-scroll.el, but
there must be a cleaner way to implement this.

Thanks in advance.