Fix bug in CanUndo() returning true after wxTextCtrl creation

For wxMSW text controls with wxTE_RICH2 style, calling SetFont() counts
as an undoable operation, resulting in CanUndo() returning true even if
no "real" changes have been made yet.

Fix this by resetting the undo stack after creating the control using
ITextDocument::Undo().

Unfortunately this interface is not available in MinGW-32, so this fix
can't be used with it.

Closes https://github.com/wxWidgets/wxWidgets/pull/2010

Closes #17524.

tests/controls/textctrltest.cpp

@@ -1432,4 +1432,35 @@ TEST_CASE("wxTextCtrl::EventsOnCreate", "[wxTextCtrl][event]")
     CHECK( updated.GetCount() == 1 );
 }
 
+TEST_CASE("wxTextCtrl::InitialCanUndo", "[wxTextCtrl][undo]")
+{
+    wxWindow* const parent = wxTheApp->GetTopWindow();
+
+    const long styles[] = { 0, wxTE_RICH, wxTE_RICH2 };
+
+    for ( size_t n = 0; n < WXSIZEOF(styles); n++ )
+    {
+        const long style = styles[n];
+
+#ifdef __MINGW32_TOOLCHAIN__
+        if ( style == wxTE_RICH2 )
+        {
+            // We can't call ITextDocument::Undo() in wxMSW code when using
+            // MinGW32, so this test would always fail with it.
+            WARN("Skipping test known to fail with MinGW-32.");
+        }
+        continue;
+#endif // __MINGW32_TOOLCHAIN__
+
+        INFO("wxTextCtrl with style " << style);
+
+        wxScopedPtr<wxTextCtrl> text(new wxTextCtrl(parent, wxID_ANY, "",
+                                                    wxDefaultPosition,
+                                                    wxDefaultSize,
+                                                    style));
+
+        CHECK( !text->CanUndo() );
+    }
+}
+
 #endif //wxUSE_TEXTCTRL