Commit 4bb3d644 introduced a limitation
to GtkRange style properties; when stepper-spacing is > 0,
trough-under-steppers is automatically set to FALSE; this means that
setting a spacing between the steppers (e.g. the scrollbar buttons) and
the trough (i.e. the area over which the slider is free to move) would
make the buttons always get the full allocation on the !orientation
direction.
The rationale is without this limitation, you would get an area which
seems clickable, but it's actually not.

While this is true, and undesirable, for big stepper spacings, themes
that use trough-under-steppers (which is TRUE by default anyway),
might want to set smaller spacings to avoid drawing a double line between
the button and the slider borders.

To add confusion, the documentation got it flipped, i.e. it stated
setting a positive stepper-spacing would set trough-under-steppers to
TRUE (which would also make the behavior expected by commit
4bb3d644 impossible).

I don't think hardcoding either of the two limitations is a good thing.
We should let themes handle this instead, and remove this limitation. If
you want the old behavior, you can manually set trough-under-steppers to
FALSE if you set a positive stepper-spacing in your theme.

https://bugzilla.gnome.org/show_bug.cgi?id=644777