ShallowEtagHeaderFilter returns empty response body for welcome page (GET /)

When a ShallowEtagHeaderFilter bean is registered, the welcome page served at GET / returns an empty response body (content-length: 0). Requesting GET /index.html
directly works correctly and returns the full content.

Affected version: Spring Boot 4.0.5 (Spring Framework 7.0.6)

Steps to reproduce:

1. Create a Spring Boot application with spring-boot-starter-web
2. Place an index.html in src/main/resources/static/
3. Register a ShallowEtagHeaderFilter bean
4. Request GET /

Expected: The response body contains the content of index.html
Actual: The response body is empty with content-length: 0

Reproduction repository: https://github.com/LeovR/spring-boot-4-etag-filter
Run ./mvnw test — the welcomePageShouldReturnNonEmptyBody test fails, while directIndexHtmlShouldReturnNonEmptyBody passes.

This worked in Spring Boot 3 / Spring Framework 6, where ShallowEtagHeaderFilter used a plain ContentCachingResponseWrapper without the conditional caching behavior.

I am not sure whether this is a Spring Boot issue or a Spring Framework issue, since the ConditionalContentCachingResponseWrapper is part of spring-web.

Start by running ./mvnw test in the linked reproduction repository and compare welcomePageShouldReturnNonEmptyBody with directIndexHtmlShouldReturnNonEmptyBody. Then inspect ShallowEtagHeaderFilter and ConditionalContentCachingResponseWrapper in spring-web. Done means GET / returns the index.html body, matching the direct request and passing the reproduction test.