blank screen triggered on direct visit by beforeLoad calls in StrictMode with trailingSlash set to 'never' or 'always'

[SeanCassiere]

Describe the bug

This bug was found when retesting #1553.

---

The crux of the bug is that, in the right conditions, when visiting a route directly (via the URL bar) the user will only see a white/blank screen with the beforeLoad not being properly awaited.

This bug only occurs when directly visiting a URL. It doesn't occur if you navigate to the page once the application is already loaded in.

---

It requires the 3 following conditions to be met.

1. React.StrictMode must be ON.
2. Router.trailingSlash must be set to always or never.
3. An async beforeLoad function is set with some wait period (i.e. sleep(500)).

When these 3 conditions have been met and a user then directly visits a route, its beforeLoad function resolves, but the user is only shown a white/blank screen.

When React.StrictMode is turned OFF, everything works correctly. But of course, this is not a solution.

Reproduction

https://stackblitz.com/edit/github-h31abz-yta2mu?file=src%2Fmain.tsx

Steps to Reproduce the Bug or Issue

When trailingSlash is set to never.

1. Visit /posts/ and see the URL then get rewritten to /posts.
2. Observe, the loader which instantly resolves, but the beforeLoad call finishes after.
3. Observe, a white/blank screen is then shown.

When trailingSlash is set to always.

1. Visit /posts and see the URL then get rewritten to /posts/.
2. However, a loader which instantly resolves, finishes before the beforeLoad call.
3. Observe, a white/blank screen is then shown.

Expected behavior

All three trailingSlash behaviours in their loading patterns should be the same, when React.StrictMode is ON versus when it is turned off.

Currently, React.StrictMode OFF yields the expected behaviours, as opposed to when ON, the beforeLoad call is not correctly awaited and a white/blank screen is shown.

Platform

• OS: macOS
• Browser: Chrome, Arc
• TSR Version: 1.32.5

[SeanCassiere]

Retested with 1.32.13 and the described bug looks to have been patched out between 1.32.5 and now.

No more white/blank screens! both with React.StrictMode ON and OFF.