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Avalonia
Develop Desktop, Embedded, Mobile and WebAssembly apps with C# and XAML. The most popular .NET UI client technology
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
I'll throw out Avalonia for a cross-platform .NET UI framework to consider. It considers itself a "spiritual successor to WPF;" in my opinion, it's got a pretty modern feel, and looks—for the most part—the same on Linux & Windows (I don't have a Mac computer, so I can't comment on OSX.) It's still pretty new though, so there's not much info on it—besides the documentation and simply sifting through the code—like there would be with older frameworks.
As far as file managers are concerned, there are few third party ones. Files is a new FOSS file manager that looks promising, but it's slow since it is an UWP app. KDE's Dolphin has a windows port, but it lacks features and its not really usable.