Previously, I asked about using constraint layout to put two NSTableView side-by-side. Side-by-side NSTableView using StackView and Constraints Thanks to Willeke's help, I was able to achieve this using only constraints, and no StackView.
The recommendation there was:
[...] Xcode is buggy. Avoid resizing the window and/or solve layout issues by updating the frames (in Xcode 9: menu Editor -> Resolve Layout Issues -> Update Frames).
I'm still having trouble with that, unfortunately. When I run my app, it now looks fine. However, in Interface Builder, clicking "Update Frames" actually makes the layout progressively worse, such that I have to Git reset to get back to a reasonable layout. I'm worried that later, when I actually need to update my view, I will be in trouble because I am unable to touch it without breaking it.
I've tried to illustrate the issue:
I'm unsure if it's helpful, but here is my list of constraints:
What could be wrong here? Have I forgotten some obvious constraint?