It looks like the input field for Fl_Spinner has become locked to a
certain size,
causing the buttons to be much larger than the input field.
Currently I seem to get gray bars above and below the white input
field unless I
make the widget as small as the input field, which is "too small".
This happened recently somehow, as I made the below screenshots for
the docs
recently,
(i.e. the "Default" and "GTK+" in the below, which look right).
Here's what the spinner looks like now inside fluid, and the
generated program looks the same: