BookReader Preview sometimes shows a grey page artifact in 1up

Eg:
- https://archive.org/details/unknown0000vari_g5u9/mode/1up
- https://archive.org/details/unknown0000vari_b2c2/mode/1up

To Reproduce:

1. Go to one of the erroring cases in 1up mode
2. Scroll down from the top

Expected: You see the preview placeholder when you get to that page
Actual: A preview placeholder appears randomly and obstructs part of the screen.

**Context**

Can repro on FF 140 + win10

**Proposal & Constraints**

> What is the proposed solution / implementation? Is there a precedent of this approach succeeding elsewhere?

**Stakeholders**

@seabelis