For me, native means "I can integrate a platform widget in the middle of it". For instance, with Qt, GTK or wxwidgets it's entirely possible to integrate a Win32 / Cocoa / X11 component right in the middle of your app (and it's super important for instance for things such as integrating audio plugins, where the plugin only gives you a HWND or NSView and you have to draw your application Chrome around it, have it follow resizes, etc.)
replyMost toolkits, including WebUI 3.0, render widgets on their own, so you can't distinguish just on that. I'd say anything written in an interpreted language is not native, and Javascript falls into that category. Dart at least is possible to compile ahead of time, and so is C#.
replyWebViews aren't written or rendered with interpreted languages either. It is also usually not Javascript that makes browser based apps so heavy. It is almost always the whole browser stack that is making them large and memory hungry, which is mostly written in C++.
replyYou can also hook a WebView up directly to a low-level language and skip Javascript entirely, so does that mean Rust + WebView = Native?