pendingComponent replaces the mounted UI on every same-route navigation since 1.170.19 (match-loading rewrite #7805) #7986

Which project does this relate to?

Router

Describe the bug

Since @tanstack/[email protected] / @tanstack/[email protected]
(the match-loading rewrite in #7805), a navigation that only changes search
params or a path param on an already-mounted route swaps the page to the
route's pendingComponent while a parent layout's async beforeLoad re-runs.

On 1.170.18 the mounted UI stayed on screen during such navigations; the
pending component only appeared when there was nothing rendered yet (initial
load). With defaultPendingMs: 0 (pending immediately on cold start) and
defaultPendingMinMs: 250, our app now flashes a full-page spinner for at
least 250 ms on every in-app tab switch and every sibling-route switch,
because the layout's auth-guard beforeLoad does a network round trip.

Shape that reproduces it: a pathless layout route with an async beforeLoad,
a child route /projects/$projectId with validateSearch, router options
defaultPendingMs: 0, defaultPendingMinMs: 250.

Complete minimal reproducer

https://github.com/salty-flower/tsr-pending-navigation-repro

Steps to Reproduce the Bug

Automated:

1. pnpm install (the repo is pinned to 1.170.19)
2. pnpm test — both tests fail: the pendingComponent appears during a
   search-only navigation and during a param-only navigation
3. pnpm add @tanstack/[email protected] && pnpm test — both pass

Visual:

1. pnpm dev, open /projects/p1
2. Click "switch tab" (search-only) or "switch project" (param-only)
3. The whole page flashes PENDING (full-page spinner) on every click on
   1.170.19; on 1.170.18 the content stays mounted

Expected behavior

As a user, I expected an in-app navigation on an already-mounted route to keep
the current UI while beforeLoad/loaders re-run (the 1.170.18 behavior), but
on 1.170.19 the mounted page is replaced by the pendingComponent on every
such navigation.

Screenshots or Videos

No response

Platform

• Router Version: 1.170.19 (regression from 1.170.18; router-core 1.171.16 vs 1.171.15)
• OS: macOS
• Browser: Firefox (also reproduced in jsdom)
• Bundler: vite
• Bundler Version: 7.x

Additional context

1.170.20 / router-core 1.171.17 only bump dependencies, so the behavior is
unchanged there. react-router pins router-core exactly, so pinning
1.170.18 is the workaround we ship today.