> But when I debug window-recenter-region-start-position, the height is
 > set to 0.  The patch below seems to fix the problem.
[...]
 > +        (body-pixel-width (window-body-width window t))
 > +        (body-pixel-height (window-body-height window t))

You're right.  I meanwhile fixed the code to calculate how many lines to
step backwards by using 'window-text-pixel-size' there too.  So now this
should work with text scaling and varying line heights too.

If you want to test it with 'recenter-region', then a rough estimate is
that the number of lines shown after the first "L:" should be equal to
or one less than the number of lines shown after the third "L:" in each
message issued.

Thanks, martin