The "Copying files" dialog is missing some X11 window hints that indicate to window managers that it's a dialog (and not a normal window).

As a result, tiling window manager treat it as a normal window, and will make it fullscreen/split the screen, instead of treating it as it would other dialogs.

This also has some minor efectos on other types of window managers - compiz will not treat it as a dialog either.

I compared the output of xprop with other dialogs, and it seems the property "_NET_WM_WINDOW_TYPE" should be set to "_NET_WM_WINDOW_TYPE_DIALOG" instead of "_NET_WM_WINDOW_TYPE_NORMAL".