A thin border is missing on a wxTLW with wxBORDER_SIMPLE style

Description

Under wxGTK, wxQt and (wxOSX?) a wxTLW with wxBORDER_SIMPLE style doesn't display a thin border around itself as documented (see wxBusyInfo image below):

wxBORDER_SIMPLE: Displays a thin border around the window.

For example, the wxBusyInfo is created like this:

https://github.com/wxWidgets/wxWidgets/blob/cffa74e2bc83ab4660b6c04155e53ff591206fc7/src/generic/busyinfo.cpp#L36-L45

but as you can see in the image below, there is no border around it:

AFAICS, wxMSW is the only platform that displays this thin border (not sure about wxOSX)

FWIW, I did this as a workaround :

diff --git a/src/generic/busyinfo.cpp b/src/generic/busyinfo.cpp
index b27e568ec6..dd93a1986d 100644
--- a/src/generic/busyinfo.cpp
+++ b/src/generic/busyinfo.cpp
@@ -41,7 +41,9 @@ void wxBusyInfo::Init(const wxBusyInfoFlags& flags)
                               wxFRAME_TOOL_WINDOW |
                               wxSTAY_ON_TOP);
 
-    wxPanel* const panel = new wxPanel(m_InfoFrame);
+    wxPanel* const panel = new wxPanel(m_InfoFrame, wxID_ANY,
+                                       wxDefaultPosition, wxDefaultSize,
+                                       wxTAB_TRAVERSAL | wxSIMPLE_BORDER);
 
     wxBoxSizer* const sizer = new wxBoxSizer(wxVERTICAL);

and this is the result:

Platform and version information

• wxWidgets version you use:
• wxWidgets port you use:
• OS and its version:
  • GTK version:
  • Which GDK backend is used:
  • Desktop environment :
  • Current theme: