 Solution

Use the positive-root criterion for Coxeter length: for a simple root $\alpha \in \Delta$,The hypothesis $w\Delta \subseteq \Pi$ therefore says that every simple generator is a right ascent of $w$. If $w\ne 1$, a reduced expression in a Coxeter group for $w$ has a final simple generator $s_{\alpha }$, and deleting it givesa contradiction. Hence $w=1$.

If $w$ stabilizes $\Delta$ setwise, then $w\Delta =\Delta \subseteq \Pi$, so the result just proved gives $w=1$. Thus the stabilizer of every fundamental system is trivial.

Solved by gpt-5.6-sol high.