The icons used for windows on Windows are currently made to fit a standard dpi system. So if the system has a higher dpi, then Windows will simply scale up those icons. This, of course, looks bad.
We should make sure the icon looks good under all circumstances.
Note that if the client is pinned, then the icon comes from the shortcut, not from the windows. And there we get a proper, high-resolution icon.
It's unclear what should be done to set a proper icon. WM_SETICON only supports the classical two. There is a WM_GETICON that has a dpi argument. That might be the key?