Thank you for your report and PR Patrick.

I agree, it would be better to to set the "type" attribute to "dialog" for the FileDialog window. Tk does it for its dialog windows. But it is more complex.

* It should be set not only for FileDialog, but for other dialog windows implemented in Python too (including dialog windows specific for IDLE). I am in process of working on making dialog windows implemented in Python looking and behaving closer to dialog windows implemented in Tk. The hardest part is writing tests for this.

* This code should be executed only on X Window. On macOS and Windows different code is needed. From Tk sources of ::tk::dialog::file::Create and ::tk::dialog::color:: :

    if {[tk windowingsystem] eq "x11"} {wm attributes $w -type dialog}

And in ::tk_dialog and ::tk::MessageBox :

    if {$windowingsystem eq "aqua"} {
        ::tk::unsupported::MacWindowStyle style $w moveableModal {}
    } elseif {$windowingsystem eq "x11"} {
        wm attributes $w -type dialog
    }

And ::tk::dialog::error::bgerror uses moveableAlert instead of moveableModal.