Hybrid Hosted WebAssembly/Server-side hosting mode

[omcnoe]

It's great that it's so easy to swap building an application between Hosted WebAssembly & Server-side, the feature works really well.

But it's a shame that it's an all or nothing choice for the entire application. I can think of some use cases where some hybrid of the two modes would be useful. For example, blocking some very expensive functionality from executing server-side in an otherwise server side app, for backend performance/hosting cost reasons. Or running some functionality server side in an otherwise hosted wasm app, if it's too much of a performance hit on clients.

Or even an automatic hybrid mode that runs simultaneously on client & server, and uses response from whichever is able to complete faster.

[Tarmil]

We could easily make it so that you can pass a config to rootComp to specify the hosting mode of an individual component. But unfortunately, Blazor itself is not amenable to this. The page must include blazor.server.js or blazor.webassembly.js, and they aren't compatible. They both define differently the same JavaScript globals. So we can't mix both modes on the same page.

We might be able to make it so that some pages have server-side components and other pages have wasm components, but even then I'm not sure that the compile-time tooling will follow.