Summary
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Rhino 6 for Mac
Version 6 (6.18.19266.13352, 2019-09-23)
I have some geometry that is imported from a SketchUp 2018 file, and selecting grouped objects is not behaving as expected. The groups consist of blocks and sub-groups, as many of the objects are nested SketchUp components and groups. Depending on which object I pick in the group, I may get the entire group or just a subset of the group. It is acting as though it selects the hierarchy at and below the object that I pick, and it seems also to be dependent on the “pick ray” that I use… I can rotate the view and click on a sub-group where the pick ray only intersects the sub-group and Rhino will only select that sub-group, but if I rotate the view where the pick ray also intersects more of the parent group, Rhino will select more of the group’s hierarchy.
I have tried to reproduce the behavior in Rhino 6.18 by creating new objects, making blocks, replicating and grouping them, but I am unable to reproduce the behavior this way. So I assume that it is an artifact of the import process from the SketchUp file that is creating the Rhino objects with some odd properties.
I’m attaching a sample model with group that exhibits this behavior. The model also contains the simple blocks that I used to try and duplicate the behavior a few different ways, so they can be ignored. Focus on the group with the curved surface and try selecting the group from different view points. The group represents a frame with a curved panel that consists of four parts, and a mounting frame that also has four parts. The objects were created in SketchUp as components that were mirrored to make symmetrical objects, and they were then made into components and/or groups as needed. I can select the mounting frame separate from the frame with curved panel, depending on which viewpoint I click from. This behavior is certainly not expected 
grouping test.3dm (4.1 MB)