would this cause problems in wayland?

No, because this is X11-specific code that is never run on Wayland and not built if X11 is not available at build time.

Now, we don't have a solution yet for dragging Wayland windows by reference. Whether we can find a solution that works on both or need different codepaths: Time will tell. For now this is just something we can do on X11 but not on Wayland, like many others - and it's not a new thing, this is old code just moved here.

Note that once the Pager is ported to libtaskmanager both sides will go through libtaskmanager and the TM appley already uses a generic data role (AbstractTasksModel::MimeData) when starting a drag, so we'll have some freedom to design a solution.

For now it's just keeping something already working working while allowing us to drop liblegacytaskmanager and make forward progress :)