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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.
Yes, but the portable GUI frameworks by Microsoft themselves are generally not very good, and they tend to be abandoned after a couple of years.
Avalonia is developed outside of the Microsoft corporate madness and seems to be slowly becoming the defacto cross-platform framework because it is expected to last a bit longer than a manager's attention span: https://avaloniaui.net/
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.