Thank you to report this wrong behavior. In fact, the Part of base plan option doesn't work correctly for any group of objects (imported or not), because I forgot to take into account groups when I added this option in the version 3.0.
This bug is fixed in the latest beta of the version 3.1.
By the way, this version manages also correctly auto selection in the text fields of color choosers and is able to highlight the current search in the help pages. A nice feature I hope you'll like!
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Emmanuel Puybaret, Sweet Home 3D creator
