Links to a mounted page in a page tree with a different domain are not prepended with the domain
If you link to a page in another page tree with a different domain, the link gets prepended with the domain of the other page tree. But if the target page mounts another page, then the domain is not prepended. It seems like, as soon as the link has the GET parameter MP, the rootline check is not done.
This happens on typolinks, but also in menus and links from the RTE. Tested TYPO3 Versions are 6.2, 7.6 and 8.7.
A detailed testcase is attached.