Briefly: no, it's a custom reactive UI framework similar to Jetpack Compose. Rust is used for native operations such as accessing file system, launching processes, etc
Interesting that you would use Rust for those operations since the JVM was already presumably giving you cross-platform implementations. Was that a significant performance bottleneck that got alleviated by using Rust, or was there another reason?
Whole architecture of Fleet is distributed, so we need a small binary without dependencies that could be uploaded to the dev container. All the access goes through that binary (even in local case)
It's interesting and says something about the state of tooling that, in response to those requirements, it was easier to switch to an entirely different language and ecosystem than it was to build off the existing work and convince the toolchain you were using to produce output with those properties.
Well, you can make standalone binaries with both Kotlin and Java these days, so this may be related to when Fleet was started rather than how you'd do it in 2021.