Add GeometryReader implementation

[MaxDesiatov]

This is just an empty API at the moment. I hope it can be implemented purely in the deferredBody of GeometryReader with the ResizeObserver API without requiring any tweaks in the Renderer protocol or the reconciler.

Seems like I need the domRef modifier that writes JSObjectRef to a given binding working first, as discussed in #231.

[carson-katri]

I can work on implementing the features outlined in that issue

[MaxDesiatov]

I started some preliminary work on that, or do you have some code already?

[carson-katri]

No, I hadn’t started yet.

[MaxDesiatov]

No worries, I'll try to get some draft of that today.

[MaxDesiatov]

This is working for me now in isolated example code, but layout bugs exposed by our demo code prevent this from being added to the demo. @carson-katri @j-f1 lmk if you think it would be fine merging this as is and adding a GeometryReader demo in a separate PR (smaller PRs is what I prefer personally).

[j-f1]

IMO if it's a small bug or something that prevents building it shouldn't be merged in general. But if it's more complex or harder-to-solve bug, it's ok to merge then follow up with an issue or PR.

[carson-katri]

What's the layout bug preventing a demo?

[MaxDesiatov]

If you launch the demo in this branch in browsers and navigate to the GeometryReader item you'll see that the size text doesn't change if you change the size of the viewport (I resized the browser window). It does change in the native demo though.

As far as I understand, the destination view of NavigationView doesn't fill all available space currently, maybe it's fixed with the _FlexFrameLayout changes made in #241?

[j-f1]

I got that to work after I added this CSS to GeometryReader:

display: flex;
align-items: center;
justify-content: center;

…weirdly, the text isn’t exactly centered.

[MaxDesiatov]

These styles worked only in Safari for me and only for height, width always stays 300. I've pushed these changes for the lack of a better option, will have a look later if #241 could resolve it.

[carson-katri]

Works great!