The "Strands" settings is a BlockBut and never return B_MATPRV like the others buttons, because of this,
never call BIF_preview_changed.
It's posible two fix for this:
1) make all the buttons into the "Strands" panel return B_MATPRV (this modify the "work mode" of the panel)
2) Add a call to BIF_preview_changed to the end of strands_menu function (as do_matbuts).
The problem with the first options, is that the panel "close" when you press/release some of the buttons, so
to preserve the current "work mode" of the panel, i choose the second option.
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