Why does the GUI block?

[tangent]

I was hoping to use Wings as a demonstration that you can have a GUI app that does not block when it's busy doing something heavy in the background without using low-level OS threads directly, but my attempt to produce a demonstration case failed horribly. (That is, I wanted to prove the value of Erlang-style concurrency vs. traditional "GUI thread" style concurrency.)

Steps to reproduce:

1. Create a cube in a new scene
   
2. Rubber-band select the cube
   
3. Press "S" five times to make lots o' polys
   
4. Right-click, Bevel
   
5. Try to do something with the menus while it munches on step 4
   

This test was with Wings 3D 2.1.5 freshly-installed onto a late 2012 iMac running macOS 10.12.3, then repeated in a Windows 10 VM under VMware Fusion on the same hardware.

I don't know if the fact that the bevel step takes many minutes is also a bug. I didn't expect to be able to bog it down so easily. (6,144 polys!)