diff --git a/lcl/interfaces/gtk2/gtk2wsforms.pp b/lcl/interfaces/gtk2/gtk2wsforms.pp
index 2d78547354..7767f7007c 100644
--- a/lcl/interfaces/gtk2/gtk2wsforms.pp
+++ b/lcl/interfaces/gtk2/gtk2wsforms.pp
@@ -775,6 +775,16 @@ begin
             gtk_window_set_keep_above(GtkWindow, True);
       end;
     end;
+
+    if AWinControl.HandleObjectShouldBeVisible and
+      not (csDesigning in AForm.ComponentState) and
+      not (AForm.FormStyle in fsAllStayOnTop) and
+      not (fsModal in AForm.FormState) and
+      (AForm.PopupMode = pmExplicit) and
+      (AForm.PopupParent = nil) then
+    begin
+      SetPopupParent(AForm, AForm.PopupMode, AForm.PopupParent);
+    end;
     {$ENDIF}
 
     Gtk2WidgetSet.SetVisible(AWinControl, AForm.HandleObjectShouldBeVisible);
@@ -846,7 +856,11 @@ begin
     pmAuto:
       PopupParent := Screen.ActiveForm;
     pmExplicit:
+    begin
       PopupParent := APopupParent;
+      if PopupParent = nil then
+        PopupParent := Application.MainForm;
+    end;
   end;
   if PopupParent <> nil then
     gtk_window_set_transient_for({%H-}PGtkWindow(ACustomForm.Handle), {%H-}PGtkWindow(PopupParent.Handle))
diff --git a/lcl/interfaces/qt/qtwidgets.pas b/lcl/interfaces/qt/qtwidgets.pas
index 55628eba96..e733d602f3 100644
--- a/lcl/interfaces/qt/qtwidgets.pas
+++ b/lcl/interfaces/qt/qtwidgets.pas
@@ -6696,13 +6696,25 @@ begin
       if Screen.ActiveForm <> nil then
         NewParent := TQtWidget(Screen.ActiveForm.Handle).Widget;
     pmExplicit:
+    begin
       // parent is FPopupParent
       if FPopupParent <> nil then
-        NewParent := FPopupParent;
+        NewParent := FPopupParent
+      {$IFDEF HASX11}
+      else
+      begin
+        if not IsMainForm then
+        begin
+          NewParent := TQtMainWindow(Application.MainForm.Handle).Widget;
+          setWindowFlags(windowFlags or QtSheet);
+        end;
+      end;
+      {$ENDIF}
+    end;
   end;
-  if (NewParent = nil) and not FShowOnTaskBar and not IsMainForm then
-    NewParent := TQtMainWindow(Application.MainForm.Handle).Widget;
-
+  if (NewParent = nil) and (FPopupMode <> pmNone) and
+    not FShowOnTaskBar and not IsMainForm then
+      NewParent := TQtMainWindow(Application.MainForm.Handle).Widget;
   ChangeParent(NewParent);
 end;