fix(ui): always render tabs in insights/frontend to avoid flash during first navigation

[TkDodo]

In the current UI, wherever we use top-level tabs, we can see a full flash of an empty page when doing the first navigation, which is pretty annoying and not great UX:

Screen.Recording.2025-10-03.at.22.18.43.mov

The technical problem is that we lazy-load each route, but each route is also responsible for rendering its own tabs. That means when we navigate through the tabs, the tabs unmount.

This PR introduces a solution where we use nested layouts to render the tabs instead of having the tabs be part of each separate page.

That means the tabs will stay mounted while only its content lazy-loads, which leads to a lot better UX:

Screen.Recording.2025-10-03.at.22.17.17.mov

Technically, we render a layout.tsx file at the root of /frontend, which will render the <FrontendHeader> for all sub-routes plus an <Outlet> for the children.

The problem is that the layout must know which “child” it is going to render in order to select the current tab.
With useMatches, we can take a look at what renders at runtime, and then “inspect” the last match to see what child it is. We could just look at the url path segment but that’s a bit brittle, so I opted for using route handles.

Route handles are often used to build dynamic UI elements like breadcrumbs based on the route hierarchy. Here, we just use it to pass custom information from the route tree (where we define our routes) to the layout, so that it knows what to do. In this case, we pass the ModuleName of the rendered page so that we can pass that to the <FrontendHeader> component. This sadly isn’t type-safe (react-router doesn’t do type-safety like that), but I think this is a fine trade-off.

[TkDodo]

note: there are sub-routes that do not get the module passed, and in those cases, we don’t want to render the tabs at all. This basically applies to all detail pages you can get to from the top landing pages.

The check is done with ’module’ in handle because for the Overview page, we need to explicitly pass handle: undefined because there is no explicit ModuleName for the Overview page.

[natemoo-re]

Massive improvement, thank you for tackling this! Curious about naming but only if this is a pattern that's going to start proliferating. If this is intended as more of a temporary/migration step, I think we should add a comment to that effect.

handle is the official name in react router for this: reactrouter.com/how-to/using-handle

[natemoo-re]

Would love someone from @getsentry/insights to weigh in before you merge, but thrilled to see this 🙌