openide/platform/navbar

Why compatibility modules exist

In short, compatibility modules solve the circular dependency problem.

1. lang-impl module is loaded with the core classloader, which is a parent of all other module classloaders => lang-impl cannot depend on module classloaders.
2. Modules intellij.platform.navbar, intellij.platform.navbar.frontend, intellij.platform.navbar.backend are still used from within lang-impl => they have to be in the same classloader as lang-impl => they cannot be loaded as separate modules.
3. Some navbar-related code depends on lang-impl, because we have to keep APIs in lang-impl, for example, com.intellij.ide.navigationToolbar.NavBarModelExtension. This code is not used from lang-impl, so it was extracted it. These are compatibility modules.

It's not possible to move everything to intellij.platform.navbar, intellij.platform.navbar.frontend, intellij.platform.navbar.backend because they would depend on lang-impl and lang-impl would depend on them.

After there is no more usages of classes from intellij.platform.navbar, intellij.platform.navbar.frontend, intellij.platform.navbar.backend in lang-impl, it's possible to remove the dependency on these modules from lang-impl. After that, they can be removed from the core classloader and become true modules. After that, the content of compatibility modules can be merged into corresponding navbar modules.