Flickering rocks in background of hidden game (logo easter egg)

### Short summary

Background rocks flicker in the hidden easter egg game accessed via the app logo, after jumping up a few platforms.

### Affected version or release

v1.1.10

### Installation context

AppImage installation on Linux amd64, Wayland session. Visual glitch scoped to the hidden game screen only.

### What happened?

After clicking the app logo to open the hidden game, landing on the first platform, scrolling down so the prompt input is out of view, and jumping across a few platforms, the rocks in the background start flickering.

### Steps to reproduce

1. Open the app
2. Click the app logo to open the hidden game
3. Land on the first platform
4. Scroll down so the prompt input field is out of view
5. Jump across a few platforms to gain some height
6. Rocks appear in the background
7. The rocks start flickering

### Expected behavior

Background rocks should render normally and not flicker.

### Additional context

_No response_

Start at the app-logo entry point for the hidden game and reproduce the flickering after scrolling the prompt out of view and jumping across platforms. Trace the background-rock rendering path on Linux Wayland, then verify that the rocks remain stable in the same sequence and that the hidden game still renders normally.

索引モデルが issue の本文から書いたものです。