Coding style nitpick: namespaces have the opening brace on a new line.

Coding style nitpick: the kdelibs coding style doesn't say anything about indenting access modifiers, but in KWin, we usually put access modifiers on the start of a line (i.e. they are not indented).

*KWin-side

Minor nitpick: you could also use uniform initialization, e.g.

int rowCount(const QModelIndex &parent = {}) const override;

It saves typing extra characters, also looks neater, IMHO. I didn't test it, so take my words with a grain of salt.

Feel free to ignore this one: as an alternative, we could use an enum instead of the boolean trap.

For better readability, you could create a struct to represent the state on both sides, e.g.:

struct State
{
    QStringList desktops;
    QHash<QString, QString> names;
    int rows;
};

State m_clientState;
State m_serverState;