Make wxSizer::SetSizeHints() work again

This function was broken when it was called for a window which was not
the window the sizer was associated with since the recent (pre-3.1.5)
changes trying to work around the problem with the initial windows size
when using GTK 3, see 9c0a8be1dc (Merge branch 'gtk-initial-size',
2021-04-13).

Fix it by passing the sizer to use for calculating the size explicitly
to WXSetInitialFittingClientSize() when we have it, and only falling
back on the window's own sizer if we don't.

Closes #19170.

include/wx/window.h

@@ -1571,7 +1571,11 @@ public:
     // that we really need to use is not known until the window is actually
     // shown, as is the case for TLWs with recent GTK versions, as it will
     // update the size again when it does become known, if necessary.
-    virtual void WXSetInitialFittingClientSize(int flags);
+    //
+    // The optional sizer argument can be passed to use the given sizer for
+    // laying out the window, which is useful if this function is called before
+    // SetSizer(). By default the window sizer is used.
+    virtual void WXSetInitialFittingClientSize(int flags, wxSizer* sizer = NULL);
 
         // get the handle of the window for the underlying window system: this
         // is only used for wxWin itself or for user code which wants to call

src/common/sizer.cpp

@@ -1079,7 +1079,7 @@ wxSize wxSizer::Fit( wxWindow *window )
     wxCHECK_MSG( window, wxDefaultSize, "window can't be NULL" );
 
     // set client size
-    window->WXSetInitialFittingClientSize(wxSIZE_SET_CURRENT);
+    window->WXSetInitialFittingClientSize(wxSIZE_SET_CURRENT, this);
 
     // return entire size
     return window->GetSize();
@@ -1119,7 +1119,8 @@ void wxSizer::SetSizeHints( wxWindow *window )
 {
     // Preserve the window's max size hints, but set the
     // lower bound according to the sizer calculations.
-    window->WXSetInitialFittingClientSize(wxSIZE_SET_CURRENT | wxSIZE_SET_MIN);
+    window->WXSetInitialFittingClientSize(wxSIZE_SET_CURRENT | wxSIZE_SET_MIN,
+                                          this);
 }
 
 #if WXWIN_COMPATIBILITY_2_8

src/common/wincmn.cpp

@@ -999,11 +999,17 @@ wxSize wxWindowBase::WindowToClientSize(const wxSize& size) const
                   size.y == -1 ? -1 : size.y - diff.y);
 }
 
-void wxWindowBase::WXSetInitialFittingClientSize(int flags)
+void wxWindowBase::WXSetInitialFittingClientSize(int flags, wxSizer* sizer)
 {
-    wxSizer* const sizer = GetSizer();
+    // Use the window sizer by default.
     if ( !sizer )
-        return;
+    {
+        sizer = GetSizer();
+
+        // If there is none, we can't compute the fitting size.
+        if ( !sizer )
+            return;
+    }
 
     const wxSize
         size = sizer->ComputeFittingClientSize(static_cast<wxWindow *>(this));