So far this is the only thing which it's not that great to work with in your extension, adding dpcov.directory manually is a little troublesome you see for instance I use Intellij Idea which let's you run tests with its ui, adding this option every time manually not only is time consuming but it causes a lot of errors by different members of the team.

I'm not really familiar with how php extensions are written but if it is possible to read phpunit.xml config directly in your plugin, using the coverage array of includes could easily get the highest root and that could be set as default, that way we can have a better default directory, don't you think so? Xdebug doesn't have this problem as I tested so it's doing something like that.

Example :
#37
#34

and many more like one of mine :

    <coverage processUncoveredFiles="true" includeUncoveredFiles="true">
        <include>
            <directory suffix=".php">./app/Http/Controllers</directory>
            <directory suffix=".php">./../../../dir/plugins/store_management</directory>
        </include>
        <exclude>
        </exclude>
    </coverage>