This was reported for the FCurve modifier restrict ranges, but might fail
elsewhere, too. Reason is that the post_but has its range (hardmin/
hardmax etc) set before the updates to the active button take place, so
changes here dont end up on the post_but (even though the RNA range
function is properly called for a new defined button - new one is not the
same as the post_but though).
Took quite a while to figure that out, and it might not even be the best
solution, but now update the ranges on the post_but when that gets
active.
Fixes T78763