-#include "wx/treectrl.h"
-#include "wx/validate.h" // always include, even if !wxUSE_VALIDATORS
+
+// this one is included by exactly one file (mdi.cpp) during wx build so even
+// although we keep it here for the library users, don't include it to avoid
+// bloating the PCH and (worse) rebuilding the entire library when it changes
+// when building the library itself
+#ifndef WXBUILDING
+ #include "wx/mdi.h"
+#endif
+
+// always include, even if !wxUSE_VALIDATORS because we need wxDefaultValidator
+#include "wx/validate.h"