Make client size computation more parallel to OnPaint() code

Make it more clear that we do what we do in DoGetBestClientSize()
because of what our OnPaint() does.

Incidentally fix off by 1 (or 2 under non-MSW platforms) mismatch
between the 2 methods: the gap between the button and the text is
actually just 2 DIPs, not 4, but we need to add another 1 DIP for the
focus rectangle under MSW.

src/generic/collheaderctrlg.cpp

@@ -77,17 +77,28 @@ bool wxGenericCollapsibleHeaderCtrl::Create(wxWindow *parent,
 
 wxSize wxGenericCollapsibleHeaderCtrl::DoGetBestClientSize() const
 {
-    wxClientDC dc(const_cast<wxGenericCollapsibleHeaderCtrl*>(this));
-    wxSize btnSize = wxRendererNative::Get().GetCollapseButtonSize(const_cast<wxGenericCollapsibleHeaderCtrl*>(this), dc);
+    wxGenericCollapsibleHeaderCtrl* const
+        self = const_cast<wxGenericCollapsibleHeaderCtrl*>(this);
+
+    // The code here parallels that of OnPaint() -- except without drawing.
+    wxClientDC dc(self);
+
+    wxSize size = wxRendererNative::Get().GetCollapseButtonSize(self, dc);
+
     wxString text;
     wxControl::FindAccelIndex(GetLabel(), &text);
-    wxSize textSize = dc.GetTextExtent(text);
-    // Add some padding if the label is not empty
-    if ( textSize.x > 0 )
-        textSize.x += FromDIP(4);
 
-    return wxSize(btnSize.x + textSize.x,
-        wxMax(textSize.y, btnSize.y));
+    const wxSize textSize = dc.GetTextExtent(text);
+
+    size.x += FromDIP(2) + textSize.x;
+    if ( textSize.y > size.y )
+        size.y = textSize.y;
+
+#ifdef __WXMSW__
+    size.IncBy(1);
+#endif // __WXMSW__
+
+    return size;
 }
 
 void wxGenericCollapsibleHeaderCtrl::SetCollapsed(bool collapsed)