Related: rhbz#1281906 set a min size on un-resizeable non-layout dialogs

a min size equates to a size-request which is reliable under gtk3+wayland
as the dialog contents size.

a default size equates to the size of the dialog, which includes decorations
under wayland

Change-Id: I20baf00fb5952ab93628f4dd6891779ce682783c

vcl/source/window/syswin.cxx

@@ -1135,6 +1135,10 @@ void SystemWindow::DoInitialLayout()
         mbIsCalculatingInitialLayoutSize = false;
         mbInitialLayoutDone = true;
     }
+    else if (!(GetStyle() & WB_SIZEABLE))
+    {
+        SetMinOutputSizePixel(GetSizePixel());
+    }
 }
 
 void SystemWindow::doDeferredInit(WinBits /*nBits*/)