I'm not sure what Kdelibs coding style says about namespace comments, but

namespace KWin
{
namespace Foo
{

// many lines of code

} // namespace Foo
} // namespace KWin

IMHO is much readable. If you're okay to have such namespace comments, maybe namespace comments should be updated in one go.

Personal opinion: I think such comments like namespace or if after a closing braces don't add any useful information. If I need to know the scope the brace ends I use the editors functionality to highlight it.