Allow using sizers for laying out wxMDIClientWindow in wxMSW.

Let the user code put wxMDIParentFrame::GetClientWindow() into a sizer and
manage it as any other window, instead of having to do it manually.

Closes #16196.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76473 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -69,6 +69,7 @@ wxMSW:
 - Add support for saving 256*256 32bpp ICOs in PNG format (Artur Wieczorek).
 - Keep menu item icon after removing and adding it back (Artur Wieczorek).
 - Add wxThread::MSWGetHandle() (troelsk).
+- Allow using sizers for laying out wxMDIClientWindow (Artur Wieczorek).
 
 wxOSX/Cocoa:
 

src/msw/mdi.cpp

@@ -38,6 +38,7 @@
     #include "wx/settings.h"
     #include "wx/intl.h"
     #include "wx/log.h"
+    #include "wx/sizer.h"
     #include "wx/toolbar.h"
 #endif
 
@@ -467,12 +468,17 @@ WXHMENU wxMDIParentFrame::MSWGetActiveMenu() const
 
 void wxMDIParentFrame::UpdateClientSize()
 {
-    if ( GetClientWindow() )
-    {
-        int width, height;
-        GetClientSize(&width, &height);
+    int width, height;
+    GetClientSize(&width, &height);
 
-        GetClientWindow()->SetSize(0, 0, width, height);
+    if ( wxSizer* sizer = GetSizer() )
+    {
+        sizer->SetDimension(0, 0, width, height);
+    }
+    else
+    {
+        if ( GetClientWindow() )
+            GetClientWindow()->SetSize(0, 0, width, height);
     }
 }