OK. So... That version fixes the startup, and my original zoom snap issue.
This following feedback on the hiding stuff is kinda long, but please take it as constructive, and absolutely supportive. This is excellent stuff.
The hiding feature does something kinda weird for me. My furniture hides and appears constantly as I rotate. It looks like anything closer than the point of rotation. I thought this was supposed to only affect walls, but I guess it makes sense for objects too. But then walls hide differently. They hide if they are in the direct line between camera and point of rotation. This is different to how objects work, so it feels odd. I have a straight wall composed of segments. If you pan sideways, only the segments directly between the point of rotation and the camera hide. Maybe walls should behave like the objects? If any end of a wall segment is closer than the point of rotation, hide it?
If a single object is the selection, it is visible. If a grouped object or an extended selection of objects are selected, then elements within those groupings can disappear too. In this case I would suggest all selected objects, or sub objects of a selected grouped object are always visible.
The last oddity... Yeah, I know I'm a PITA. I've gotten used to selecting a room floor and rotating around it. If I do this, much of the furniture disappears in that room. Rather than using the object/floor/wall center point, find the closest point of that selection, and anything forward of that point gets hidden. Anything behind that point is visible. This might not work well because of complex room shapes, so is just brainstorming.
With the polyline you're getting really close to having a useful sectioning tool. The only improvements I could suggest there are the ability to set the camera in standard orthoganol positions, and the ability to remove the perspective from 3D view. Do you also get the following? When selecting a polyline not displayed in the 3D view, the view centers nowhere near the polyline, possibly the 0,0 origin. It centers properly is the polyline is displayed in the 3d view. This has always been the case in SH3D, and nothing to do with your plugin.
I can post another video if you want a better visualisation.
