For dialog without explicitly SetMinSize, we should use SetSizerAndFit instead, otherwise the dialog will not show correctly on GTK3. (OrcaSlicer/OrcaSlicer#14561)

- and if `SetSizer` is called before the full layout has been built, then an extra `SetSizeHints` should be called before layout/fit so the min size can be properly set automatically based on children's min sizes accordingly.

src/slic3r/GUI/NetworkPluginDialog.cpp

@@ -47,6 +47,7 @@ NetworkPluginDownloadDialog::NetworkPluginDownloadDialog(wxWindow* parent, Mode
     } else {
         create_missing_plugin_ui();
     }
+    main_sizer->SetSizeHints(this);
     Layout();
     Fit();
     CentreOnParent();

src/slic3r/GUI/Plater.cpp

@@ -15094,7 +15094,7 @@ ProjectDropDialog::ProjectDropDialog(const std::string &filename)
 
     m_sizer_main->Add(dlg_btns, 0, wxEXPAND);
 
-    SetSizer(m_sizer_main);
+    SetSizerAndFit(m_sizer_main);
     Layout();
     Fit();
     Centre(wxBOTH);