Fix drawing of static box with empty label under MSW

The top part of the box border was not drawn after the changes of
900c6d5d75 which decreased the top border
size too much in the case of empty label.

Fix this and also try to improve the code a little bit by introducing
another symbolic constant in MSW code and not using the constant from
the base class code, as like this we can at least keep all the related
constants together -- even if they're still as magic as ever.

Closes #18191.

src/msw/statbox.cpp

@@ -60,6 +60,10 @@ const int LABEL_HORZ_OFFSET = 9;
 const int LABEL_HORZ_BORDER = 2;
 const int LABEL_VERT_BORDER = 2;
 
+// Offset of the box contents from left/right/bottom edge (top one is
+// different, see GetBordersForSizer()). This one is completely arbitrary.
+const int CHILDREN_OFFSET = 5;
+
 } // anonymous namespace
 
 // ----------------------------------------------------------------------------
@@ -192,10 +196,29 @@ wxSize wxStaticBox::DoGetBestSize() const
 
 void wxStaticBox::GetBordersForSizer(int *borderTop, int *borderOther) const
 {
-    wxStaticBoxBase::GetBordersForSizer(borderTop, borderOther);
+    // Base class version doesn't leave enough space at the top when the label
+    // is empty, so we can't use it here, even though the code is pretty
+    // similar.
+    if ( m_labelWin )
+    {
+        *borderTop = m_labelWin->GetSize().y;
+    }
+    else if ( !GetLabel().empty() )
+    {
+        *borderTop = GetCharHeight();
+    }
+    else // No label window nor text.
+    {
+        // This is completely arbitrary, but using the full char height in
+        // this case too seems bad as it leaves too much space at the top
+        // (although it does have the advantage of aligning the controls
+        // inside static boxes with and without labels vertically).
+        *borderTop = 2*FromDIP(CHILDREN_OFFSET);
+    }
 
-    // need extra space, don't know how much but this seems to be enough
     *borderTop += FromDIP(LABEL_VERT_BORDER);
+
+    *borderOther = FromDIP(CHILDREN_OFFSET);
 }
 
 bool wxStaticBox::SetBackgroundColour(const wxColour& colour)