Discovered while testing MC-268876.

The bug

If a resource pack modifies an existing shader, enabling that resource pack will not change that shader. F3+T will be required to reload resource packs, and only after this point will the shader changes manifest.

How to reproduce

1. Change menu background blurriness to 100% in Accessibility Settings
2. Download and apply the attached resource pack
3. Observe how the blurriness is the same as before in the game menu
4. Exit the menu, then use F3+T
5. Re-enter the game menu
6. Observe how the blur is now different

Expected results

The changes to the blur would apply immediately after activating the resource pack.

Actual results

They are not - a force reload is required.