> But do you really need that as a requirement across the boars for all apps?
No, there are a few exceptions, but mostly yes. And I'm sure more than just the exceptions will use Flutter which is why I'm not looking forward to it.
> How many times have you wanted to save an image or copy text from a UI element in a native app?
Often. I mean not necessarily from a UI element but a lot of the time I want to get an image from a native app and can't.
> How many times have you been frustrated in a web app that when you search using the browser you are getting results from UI elements or “app chrome” instead of the actual content that the web app is displaying?
Almost never. There are rarely many extra results. In fact I already am bothered far more often when websites hijack Ctrl-F (I do not understand why browsers allow this) and fail to search the entire page. With flutter this will become the norm and I am not looking forward to it.
> Isn’t it enough to have some fields with content a user actually would want to copy/search
No, it isn't. I regularly copy things that people didn't expect. If it is just to send someone instructions how to use the app. In fact Android has automatic copy based on OCR from the app switcher and I use that a couple of times a day. There is already too much uncopyable text around. For example all messaging apps on android appear to only support copying the full message. This is what happens when you let people mess with standard UX patterns, they break them!
> You’ll likely end up spending an awful lot of time to cater for all the unexpected things that may happen when someone just clicks a link copied from the address bar
Good! I love apps with great links. It makes them so much easier to use. I don't need to go through their custom navigation to get to my favourite pages. I can use the browser navigation that I already know and love. Also I can share links to my friends! Maybe I can help people use the app better!
You just described a bunch of problems that I already have! Especially on mobile devices. This is why I hope that Flutter on web doesn't take off. We will be making all of those annoyances more common! You seem to think that you can predict all of my use cases and provide the share button manually right when I need it. However product developers on a time crunch often miss or skip use cases so it is much better to have supported by default (like text find/copy that can be disabled) than hope that a developer will think, and find time, to implement them. And if URLs encourage people to make it possible to save and share the state of their app then I think that is a good thing!