It's not needed wtih Qt category logging.
The ifdefs are from kde4 times. There it was also not needed, but nevertheless there. When working on the qt5 port I kept that under the assumption that the previous maintainer had a good reason for ifdefing it. Looking at the code I doubt it is side effect free. E.g. lines 694 following look dangerous to me. In fact I wouldn't guarantee that this won't result in a crash on wayland. The code paths have not been used in production for years.
Given that I would rather tend to just drop the ifdefs sections completely.
I'll remove the ones using m_clip, you're right that they're not const methods, so could potentially throw off any debug anyway.
The code paths have not been used in production for years.
That will also still effectively be the case here.
qCDebug() isn't a function like qDebug() but a very clever macro. The rest of the line is only evaluated if that category logging is enabled.