diff --git a/FL/fl_types.h b/FL/fl_types.h
index 51caf1a0b..631c7250b 100644
--- a/FL/fl_types.h
+++ b/FL/fl_types.h
@@ -3,17 +3,17 @@
  *
  * Simple "C"-style types for the Fast Light Tool Kit (FLTK).
  *
- * Copyright 1998-2015 by Bill Spitzak and others.
+ * Copyright 1998-2020 by Bill Spitzak and others.
  *
  * This library is free software. Distribution and use rights are outlined in
  * the file "COPYING" which should have been included with this file.  If this
  * file is missing or damaged, see the license at:
  *
- *     http://www.fltk.org/COPYING.php
+ *     https://www.fltk.org/COPYING.php
  *
  * Please report all bugs and problems on the following page:
  *
- *     http://www.fltk.org/str.php
+ *     https://www.fltk.org/str.php
  */
 
 /** \file
@@ -42,7 +42,27 @@ typedef char *Fl_String;
  */
 typedef const char *Fl_CString;
 
-/** 24-bit Unicode character + 8-bit indicator for keyboard flags */
+/** 16-bit Unicode character + 8-bit indicator for keyboard flags.
+
+  \note This \b should be 24-bit Unicode character + 8-bit indicator for
+    keyboard flags. The upper 8 bits are currently unused but reserved.
+
+  Due to compatibility issues this type and all FLTK \b shortcuts can only
+  be used with 16-bit Unicode characters (<tt>U+0000 .. U+FFFF</tt>) and
+  not with the full range of unicode characters (<tt>U+0000 .. U+10FFFF</tt>).
+
+  This is caused by the bit flags \c FL_SHIFT, \c FL_CTRL, \c FL_ALT, and
+  \c FL_META being all in the range <tt>0x010000 .. 0x400000</tt>.
+
+  \todo Discuss and decide whether we can "shift" these special keyboard
+    flags to the upper byte to enable full 21-bit Unicode characters
+    (<tt>U+0000 .. U+10FFFF</tt>) plus the keyboard indicator bits as this
+    was originally intended. This would be possible if we could rely on \b all
+    programs being coded with symbolic names and not hard coded bit values.
+
+  \internal Can we do the move for 1.4 or, if not, for any later version
+    that is allowed to break the ABI?
+*/
 typedef unsigned int Fl_Shortcut;
 
 /** 24-bit Unicode character - upper 8 bits are unused */