Spring Boot & Maven war overlay

I'm working with a Maven multi module with a war depending on an other war.

The Spring Boot webapp depends on a basic webapp that only serves html files.

When I run the Spring Boot app, I'm able to access services and html from the main webapp (the Spring Boot one) but I can't access the html files from the dependency war (404). But these html files are well packaged in the Spring Boot webapp war...

Here is a project that show the problem:
https://github.com/cthiebault/spring-boot-war-overlays

This project has 2 war modules:

dependency-webapp: basic webapp serving just html (/dependency/index.html)
main-webapp: Spring Boot webapp (copied from Spring Guides gs-serving-web-content). This webapp has a dependency on dependency-webapp.

Here is the main-webapp pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <parent>
    <groupId>spring-boot-overlays</groupId>
    <artifactId>parent</artifactId>
    <version>0.1.0-SNAPSHOT</version>
  </parent>

  <artifactId>main-webapp</artifactId>
  <packaging>war</packaging>

  <dependencies>
    <dependency>
      <groupId>spring-boot-overlays</groupId>
      <artifactId>dependency-webapp</artifactId>
      <version>0.1.0-SNAPSHOT</version>
      <type>war</type>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-thymeleaf</artifactId>
    </dependency>
  </dependencies>

  <build>
    <plugins>
      <plugin>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-maven-plugin</artifactId>
      </plugin>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-war-plugin</artifactId>
        <configuration>
          <overlays>
            <overlay>
              <groupId>spring-boot-overlays</groupId>
              <artifactId>dependency-webapp</artifactId>
            </overlay>
          </overlays>
        </configuration>
      </plugin>
    </plugins>
  </build>

  <properties>
    <start-class>hello.Application</start-class>
  </properties>

</project>

Run the webapp:

mvn install
cd main-webapp
mvn spring-boot:run

EDIT 1: Here are the logs when I try to access html pages:

http://localhost:8080/index.html --> OK

o.s.web.servlet.DispatcherServlet        : DispatcherServlet with name 'dispatcherServlet' processing GET request for [/index.html]
s.w.s.m.m.a.RequestMappingHandlerMapping : Looking up handler method for path /index.html
s.w.s.m.m.a.RequestMappingHandlerMapping : Did not find handler method for [/index.html]
o.s.w.s.handler.SimpleUrlHandlerMapping  : Matching patterns for request [/index.html] are [/**]
o.s.w.s.handler.SimpleUrlHandlerMapping  : URI Template variables for request [/index.html] are {}
o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapping [/index.html] to HandlerExecutionChain with handler [org.springframework.web.servlet.resource.ResourceHttpRequestHandler@25595861] and 1 interceptor
o.s.web.servlet.DispatcherServlet        : Last-Modified value for [/index.html] is: -1
o.s.w.s.r.ResourceHttpRequestHandler     : Trying relative path [index.html] against base location: ServletContext resource [/]
o.s.w.s.r.ResourceHttpRequestHandler     : Found matching resource: ServletContext resource [/index.html]
o.s.w.s.r.ResourceHttpRequestHandler     : Determined media type 'text/html' for ServletContext resource [/index.html]
o.s.w.s.r.ResourceHttpRequestHandler     : Resource not modified - returning 304
o.s.web.servlet.DispatcherServlet        : Null ModelAndView returned to DispatcherServlet with name 'dispatcherServlet': assuming HandlerAdapter completed request handling
o.s.web.servlet.DispatcherServlet        : Successfully completed request

http://localhost:8080/dependency/index.html --> ERROR 404

o.s.web.servlet.DispatcherServlet        : DispatcherServlet with name 'dispatcherServlet' processing GET request for [/dependency/index.html]
s.w.s.m.m.a.RequestMappingHandlerMapping : Looking up handler method for path /dependency/index.html
s.w.s.m.m.a.RequestMappingHandlerMapping : Did not find handler method for [/dependency/index.html]
o.s.w.s.handler.SimpleUrlHandlerMapping  : Matching patterns for request [/dependency/index.html] are [/**]
o.s.w.s.handler.SimpleUrlHandlerMapping  : URI Template variables for request [/dependency/index.html] are {}
o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapping [/dependency/index.html] to HandlerExecutionChain with handler [org.springframework.web.servlet.resource.ResourceHttpRequestHandler@25595861] and 1 interceptor
o.s.web.servlet.DispatcherServlet        : Last-Modified value for [/dependency/index.html] is: -1
o.s.w.s.r.ResourceHttpRequestHandler     : Trying relative path [dependency/index.html] against base location: ServletContext resource [/]
o.s.w.s.r.ResourceHttpRequestHandler     : Trying relative path [dependency/index.html] against base location: class path resource [META-INF/resources/]
o.s.w.s.r.ResourceHttpRequestHandler     : Trying relative path [dependency/index.html] against base location: class path resource [resources/]
o.s.w.s.r.ResourceHttpRequestHandler     : Trying relative path [dependency/index.html] against base location: class path resource [static/]
o.s.w.s.r.ResourceHttpRequestHandler     : Trying relative path [dependency/index.html] against base location: class path resource [public/]
o.s.w.s.r.ResourceHttpRequestHandler     : No matching resource found - returning 404
o.s.web.servlet.DispatcherServlet        : Null ModelAndView returned to DispatcherServlet with name 'dispatcherServlet': assuming HandlerAdapter completed request handling
o.s.web.servlet.DispatcherServlet        : Successfully completed request

Any idea of what's wrong?

The Spring Boot plugin doesn't know about the overlay (it's pretty basic), so you can raise an issue about that if you like in github. The WAR file from your project deploys and runs (with java -jar) just fine though, so everything else is working.