There appears to be an issue with the resizable configuration on regions within a grid, even when resizable is set to false the region can still be resized. I have attached an example of this on the "Multi-region demo" example you provide, the only thing I have changed is making the middle region none resizable.
We noticed this issue on our own grids, with an action column in its own region being resizable; changing the column or right region to resizable doesn't resolve this issue. Would it be possible to have this resolved. Thank-you.