BTW, there are TWO things that help reduce the flutter tendency of a surface.

One is the thing we've just been talking about, the squared off trailing edge.

Another is to taper the surface to a narrower trailing edge. That often works by itself, just like squaring often works by itself when modifying an existing surface. Why? Because it takes away a large ill-defined area where the top and bottom airflows have room to fluctuate. Reduce the space where fluctuation can work, and there is less pressure variances from top to bottom.

But of course, tapering also lightens. And the mass of the surface contributes greatly to what speed the flutter starts. So there are two very significant reasons to taper. Also, tapering does not reduce the area of the surface like hacking off a rounded TE can.