Alternate title: CAD beginner bites off more than he can chew.
In their online tutorial, Onshape promotes designing multiple parts in a single part studio, rather than one-part-per-file as in some traditional CAD software. This allows easy sharing of dimensions across interacting parts. When set up properly, updates can be automatically propagated across all the affected parts.
"Sounds great!" thought this Onshape beginner, "I'll put every component of my design in a single part studio!"
This was, in hindsight, too much to tackle all at once. Once I had tens of parts scattered across different planes of reference, I was starting to lose track of what I'm doing. Which had the (amusing in hindsight) result that I started confusing Onshape as well. Parts were sharing dimensions from other parts that coincidentally had the same dimension, but there was no real reason they had to be tied together. So when I wanted to change one of them, Onshape tried to automatically propagate changes across them all, and the results made no sense.
After spending too much time untangling this self-inflicted pain, I decided to take this as a lesson learned and start over.
Yes, one can design multiple parts in a single part studio, but the user still need to practice sensible partitioning of components in a large project.
The file was named "Easel PC" because I started with the idea of a minimalist frame inspired by a painter's canvas easel. I don't know if the concept will work so I didn't name the project "Easel PC". I'm aware that the easel-inspired design might change, but the luggable nature will not. That part's dictated by physics!