FreePascal + Lazarus have been desktop crossplatform for many years. “But it’s Pascal! Nobody uses Pascal! And the defaults are fugly!”, fair enough, but it offers compatible crossplatform UI with a single codebase.

Java also lets you write UI stuff and keep a single codebase for multiple platforms, thanks to the JVM. It always looks “weird” or “ugly” next to whatever OS’s default UI is and also needs a compatible JRM installed, but it works.

Nowadays, web/javascript projects can opt for Tauri or Neutralinojs instead of Electron. They use the OS’s native HTML renderer, no browser required.