[Bug]: Switching apps in Firefox flashes backround colour instead of keeping backround image

When changing between apps in Firefox (or ff based), the entire viewport briefly gets filled with the background colour.

It is not new behaviour, I just recently got reminded i should report it while testing stuff with MS Edge where the bug does not appear. As far as I remember, which I guestimate as nc 27, it had behaved that way on firefox. I have not tested webkit-based.

Here's a short clip to make it clear: expected behaviour is to not see the orange at all.
Chromium browsers don't seem to exhibit this (at least not chromium, ms edge).

https://github.com/user-attachments/assets/bcef3b91-385f-406e-8ee7-6079f09492b9

The bug gives the appearance of the pages loading slower, and depending on the "lightness" difference in the background and background colour can "lightly flash-bang" the users. It just generally looks "clunky".

Nextcloud version: 30.0.8 (but has existed since at least nc 27)
Firefox version: 136.0.1 (but has been since at least ff 115.0)
Operating system: OpenSUSE Tumbleweed (Linux; kernel 6.13.8, mesa, amd gpu)

Reproduce the app-switching transition in Firefox using the attached clip as the reference, then compare it with Edge or another Chromium browser. Trace the frontend rendering of the viewport background and identify the relevant app-switching entry point; done means the background image remains visible without an orange or other background-colour flash.