What's best Uwp alternative to Xamarin Forms' `FlexLayout`

Windows Community Toolkit offers WrapPanel and WrapLayout, while not being the exact equivalent of FlexLayout, they do cover a lot of scenarios, especially for a collection of elements. They allow for stacks of elements and automatically go on the next row/column when required.

Reference: WrapLayout and WrapPanel

Uno's support of Windows Community Toolkit is here and available on nuget.

FWIW, I've translated Xamarin.Forms.FlexLayout (including bugs) to Uno and have submitted it to as a pull request to the Windows Community Toolkit team. That being said, they asked me to rename it FlexPanel and asked for changes that requires usage with the current dev version of WCT. I don't have time at the moment to implement everything they've asked, so it's going to be a while before it will make it through the WCT review process.

If you want an implementation that doesn't require too much complexity to integrate into your projects, I have a working repo that I'm actively maintaining you are welcome to try. Alternatively, I also still have the development repo I used for the bulk of the translation and testing before trying to integrate into the WCT codebase. Again, you're welcome to try.

From what I know, there's no direct equivalent on UWP. But it should be possible to simply port the FlexLayout's code to UWP/WinUI/Uno.

On WinUI, there's a new thing called ItemsRepeater where you have a lot of flexibility for the layout of items, but it's not supported yet on Uno. You may want to follow its experimental Pull Request.