a non-fast-refresh update doesn't cause a cell reload

[jaked]

In 6738918 I made it so there's not a full-page reload for a client-side cell whenever an unrelated cell changes; but this breaks full-page reloads that are actually needed (e.g. if you change the exports of a cell).

[jaked]

Vite's HMR propagates changes bottom up. If a change is accepted by a module along the way then propagation stops; otherwise a full reload happens when propagation gets to the root of the dependency tree. This scheme doesn't account for multiply-rooted trees (e.g. multiple cells with independent dependencies), so if a change propagates to a root it sends a full reload message to all views.

To handle this correctly we need to pass the affected roots in reload messages, and check in the message handler for a view that it's one of the affected roots. (I recall this being tricky because the handler is generic and doesn't have access to the root; it has the window URL but the dependency graph doesn't connect the Javascript root module with the window URL.)