bug#18195: 24.3.92; window-screen-lines is not accurate

[martin rudalics]

> No, I mean that it is generally unreliable to measure the pixel
> dimensions of some text in one buffer, then assume that the dimensions
> will be the same when displayed as overlay in another buffer.  This is
> only true when both buffers use the same single font and the same line
> spacing; otherwise, it's simply false.

There is no other buffer.  Everything is done in the buffer where the
overlay is supposed to appear.

martin