[not bug] 3d combine and mirror surface get freeze

[wings59000]

Hello,

from https://youtu.be/RHg4A6zAeU4?t=428 i try to get the same combine and mirror but i get a freeze from wings3d that you can see here.
I have to kill processus with ctrl alt sup from win 10.
I have tried 2 times it is the same.
Here the capture video : https://youtu.be/5uOsHNwERm8

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If i just select the face with miror i don't get the cushions

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To get the good result i have to first
- select the face of the sofa and mirror to get all the sofa
- repeat this process with each cushions but they won't be symmetric