tdf#112845 get optimal size before showing popups

because in rtl mode we need to know the final size in order
to position it correctly according to the right edge

Change-Id: Ia701626968f828fd23ab1637ed6171d0e3367c16
Reviewed-on: https://gerrit.libreoffice.org/43460Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/window/floatwin.cxx

@@ -686,6 +686,7 @@ void FloatingWindow::StartPopupMode( const tools::Rectangle& rRect, FloatWinPopu
 
     // compute window position according to flags and arrangement
     sal_uInt16 nArrangeIndex;
+    DoInitialLayout();
     mpImplData->maPos = ImplCalcPos( this, rRect, nFlags, nArrangeIndex );
     SetPosPixel( mpImplData->maPos );