the step increment was set as the width of a character:
if (gtk_adjustment_get_upper(adjustmenth) != horizEndPreferred || gtk_adjustment_get_page_size(adjustmenth) != pageWidth || ...
This `adjustmenth` is the horizontal scroll adjustment, not the vertical which is `adjustmentv` set just above. The arrow step size for `adjustmentv` is not changed here with `gtk_adjustment_set_step_increment` and is left at its original 1.0.