Posting this here as a summary for the three people left writing native macOS apps using AppKit, who might stumble here from a search.
macOS 26 has brought us the yearly facelift that – as tradition – brings no break-through features or discernible improvement to usability (quite the opposite, this time around). This year’s diktat from Marketing has also brought a regression that affects how AppKit reconstructs and displays UIs in XIB/NIB files. (Haven't checked if this exists on UIKit too.) If you had carefully laid out your controls to be aligned without the assistance of auto-layout, reopening these files with Xcode 26 will show all controls to be misaligned. This same problem is sadly visible at runtime too. AppKit has removed the crucial distinction between frame rectangles and alignment rectangles. You would be spared from this problem only if your controls were participating in auto-layout by having at least one constraint, or by calling -[NSView setTranslatesAutoresizingMaskIntoConstraints:YES] and setting constraints programmatically.
The most obvious solution is to slowly add constraints to all your XIBs. It makes sense if your app uses simpler view hierarchies. For complex hierarchies, auto-layout never was a viable option. The more constraints you add to your XIB the slower it becomes, until the point where adding a single widget or constraint will take seconds. Assuming they ship with this regression (still there as of Beta 4) the solution for large XIBs is to painfully realign and resize all controls while leaving them on the older springs-and-struts layout system. This luckily can still be done fast in Xcode. The downside? All your controls will look correct on macOS 26 but will be both misaligned and probably look too small on previous versions of macOS. For the latter problem you can thank the fact that they changed the size of controls so that a small button or popup menu on macOS 26 is taller and wider than on previous versions 🤷♂️ (my previous post was incorrect on this point)
Here we are in 2025 with NextStep-era UI still being the only one you can preview and layout reliably both in Xcode and at runtime. They never allowed auto-layout to become great. Having used it both programmatically and visually, it's been left 90% done to make room for SwiftUI, now 6 years old and nowhere near usable without massive problems and frustration. I wouldn't be surprised if more apps switch to non-native UI frameworks just to stay away from this. Taking a step back from this particular regression, the broader picture is that Apple keeps externalizing upkeep costs to its third-party developers with a yearly facelift that feels more and more superficial, adding busywork to our schedule, forcing rewrites and redesigns, and bringing no rewards. Even if you jump through all of these hoops, your software gains no new features that might actually be considered valuable to users.
Best of luck!
