bug#6518: 24.0.50; window-height returns wrong value

[Christoph]

I verified this in GNU Emacs 23.2.1 (i386-mingw-nt6.1.7600) of 
2010-05-08 on G41R2F1, on Windows 7.

I decreased the window height so that only one line is visible in the 
buffer.

(window-height) function indeed returns 2.

But the documenation says:
The return value includes WINDOW's mode line and header line, if any.

Therefore, the window height of 2 is correct: 1 line in the buffer plus 
the mode line.

If I decrease the window size further that even the mode line 
disappears, (window-height) reports the correct value of 1.

The next test was setting lines-spacing to 5.

In decreased the window height so only 1 line was visible, plus the mode 
line. I checked that going to the next line indeed results in a complete 
screen scroll.

(window-height) now returns 3, which is not correct. Setting 
line-spacing back to 0 shows that 2 lines are visible, plus the mode 
line, result in a total of 3 lines.

The (window-height) function doesn't seem to take the value of 
line-spacing into account.

Christoph