Reproduction steps:
1. Open the attached “IN-104129.zip” project
2. Go to File > Build Profiles
3. Enable “Development Build” checkbox
4. Build and Run the project for WebGL
5. In the browser, click on “Load more” button ~4 times
6. Observe the Development Console and the Player

Expected result: No errors
Actual result: “ArgumentOutOfRangeException: Index was out of range.” errors are thrown, and the UITK layout is rendered incorrectly

Reproducible with: 2023.3.0a19, 6000.0.51f1, 6000.1.7f1, 6000.2.0b5
Couldn’t test with: 2022.3.62f1 (Could not find the “Enable Native C/C++ Multithreading), 2023.3.0a2 - 2023.3.0a18 (Could not build due to errors)

Reproducible on: Windows 10 (user reported), Windows 11
Not reproducible on: No other environments tested

Notes:
- Also reported in discussions: [https://discussions.unity.com/t/ui-toolkit-webgpu-browser-errors-6-2-beta/1651665/4|https://discussions.unity.com/t/ui-toolkit-webgpu-browser-errors-6-2-beta/1651665/4|smart-link]
- If the issue does not reproduce on first attempt, reload the Player and click the button a few more times
- Reproduces with both WebGL 2 and WebGPU Graphics API
- Sometimes the browser also freezes up
- Does not reproduce with Native C/C++ Multithreading disabled in the Publishing settings