I'd like the request to be reopened, because the current function
`window-inside-absolute-pixel-edges' does not provide the requested
behaviour.

Suppose no fringes, and header-line.  All considerations for a GTK build.
 
0--------------------------------------*
|           display                    |
| A----------------------------+       |
| |      emacs@foo.bar         +       |
| |-B------------------------+ +       |
| | |       toolbar          | |       |
| | +------------------------+ |       |
| | |       menubar          | |       |
| | C------------------------+ |       |
| | |                        | |       |
| | |       window           | |       |
| | |   D---------+          | |       |
| | |   | tooltip |          | |       |
| | |   +---------+          | |       |
| | +------------------------+ |       |
| *----------------------------*       |
|                                      |
*--------------------------------------*
                               

We have a buffer position D =
(D.x, D.y) in some window and want to display a tool-tip there. At the
moment,

       (posn-x-y (posn-at-point))

gives us a position relative to C, so we would need it's absolute
position.  

But `window-absolute-pixel-edges' gives the absolute position of

    (C.x - (B.x - A.x), C.y - (B.y - A.y))
  = (A.x , C.y - (B.y - A.y)) ,

that is, it does not account for the width and height (B - A) of the
window managers decorations.  Furthermore these sizes are unknown to the
lisp side, making it impossible to complete the desired task.

So it seems to me that `window-absolute-pixel-edges' should return the
absolute position of C, such that the tool-tip (or other frame) may be
positioned at C + D.

I noticed, that the frame struct already has members x_pixels_diff and
y_pixels_diff, such that

    (x_pixels_diff, y_pixels_diff) = (B.x - A.x, B.y - A.y) ,

such that we may compute C with this values.  So I propose extending
`calc_absolute_offset' by adding these pixel_diff values.  For GTK this
appears to be especially easy, since these diff values already account
for the tool-bar and menu-bar sizes.