The default type of Composer package is library. Composer puts such packages in vendor/[vendor]/[name], which does not fit typical WordPress directory layout well.

However it is easy adjust it.

Require composer/installers

composer/installers is a package of special composer-installer type. It catches packages with non-library types during installation and (if they match types it supports) adjusts their paths.

To make use of it your extension's composer.json should contain:

{
    "type"    : "wordpress-plugin",
    "require" : {
        "composer/installers" : "~1.0"
    }
}

Type can also be wordpress-theme and wordpress-muplugin.

When used in root site stack package it would make packages go into wp-content/* directories as WordPress things usually do.

Customize paths

If you don't like defaults you can override them in root package (and there only, one of things only root one gets to decide) individually or in bulk:

{
    "extra" : {
        "installer-paths" : {
            "content/plugins/{$name}/": ["type:wordpress-plugin"]
        }
    }
}

Note that this only works for packages that declare composer/installers support. You cannot customize paths for generic library packages. However you can customize location of vendor directory to rename and/or move from default root location:

{
    "config" : {
        "vendor-dir" : "wp-content/vendor"
    }
}

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