In iOS 27, willTransition(to:with:), registerForTraitChanges(), and other mechanisms to monitor size classes do not change when an iPhone interface is resized. Only viewWillTransition(to:with:) is invoked with a new size. And this only happens on the iPhone: the iPad continues to work as it has in the past.
It appears that this is intended behavior. That is not to say that it's intuitive behavior.
Many experienced developers are encountering the "horizontal size is always compact" behavior and immediately thinking "this must be a beta bug":
https://fatbobman.com/en/posts/from-size-class-to-available-space/
I get that the new size class behavior is expressing static device semantics and is no longer a dynamic size indicator. The problem is that the tools we have been using to build layouts for the past decade use size classes as dynamic sizing indicators.
Storyboards can contain variations that specify whether a constraint is used for regular or compact widths/ heights. Developers have used this ability to automatically adjust layouts as the size classes change.
In one case, I use this capability to adjust a top-over-bottom layout in a portrait configuration to a side-by-side in a landscape configuration: switching centering, leading/trailing, and aspect ratios to fit the available size. I suspect that many developers who have taken the time to create a unique layout for an iPad are doing something similar.
(Ironically, the folks who treated an iPad as a big iPhone are the ones least affected by this change.)
When iPadOS got the ability to resize interfaces (as UIScene windows), I made sure that the automatic size class contraints worked correctly and made adjustments as necessary. That work now has to be discarded and switched over to something more universal.
If this is truly the intended behavior going forward, time needs to be invested in updating the tooling behind automatic constraint variations:
- There should be warnings when the storyboard is compiled. I have dozens of automatic contraints embedded in the storyboard: and they're hard to find (each subview has its own contraints, so it's a manual traversal of a huge tree).
- There should be runtime warnings that your code that inspects size classes won't be executed. This was the biggest "what the hell is going on?" when trying to resize the interface the first time.
At a higher level, Apple engineers have described what is happening. They have not explained why the iPhone is behaving differently than an iPad. And that's the root of this whole situation being unintuitive. We've got no clue.
And developers without a clue are unlikely to adopt a new technology. It's essential that Apple explains this change in more detail. Yes, you're going to have to obfuscate it and make us read between the lines, but it's got to be done. (Everyone understands the changes regarding mainScreen wink wink, for example.)
A reply in this thread would be a good place to start this explanation.